Stock Options When Leaving a Company: The 90-Day ISO Window

For employees with ISOs or NQSOs who are leaving — voluntarily, due to layoff, or at end of contract. Decisions made in this window are largely irreversible. Not tax or legal advice; your plan document controls specifics.

What "post-termination exercise period" means

Every option plan specifies a post-termination exercise period (PTEP) — the window after your last day in which you can still exercise unvested or vested options. Two things govern the PTEP:

• Your plan document. This sets the outer limit. Read it — it controls. Some plans give 30 days; others give 90; some give a year or more, especially for NQSOs or for employees meeting age/service retirement criteria.
• The IRS ISO rules. Regardless of what your plan says for ISOs, the IRS imposes a hard 3-month window. If you exercise ISOs later than 3 months after termination, they lose ISO status automatically. They don't become worthless — they become NQSOs taxed accordingly.

The practical result: even if your plan's PTEP is 12 months, exercising your ISOs in month 4 means paying ordinary income tax on the full spread — not AMT preference treatment — plus FICA taxes you wouldn't owe on an ISO exercise.

What actually changes after 90 days: ISO vs. NQSO taxation

The difference between exercising an ISO within 3 months versus after is a real dollar difference:

For large option grants, the difference is significant. A $2M spread exercised as an ISO (and eventually sold as a qualifying disposition) vs. the same spread as an NQSO can mean $400,000–$600,000 more in federal tax on the NQSO side — simply because you missed the 90-day window.

NQSOs: a different deadline problem

NQSOs aren't subject to the ISO 90-day rule, but they're still subject to the plan's PTEP. That window is entirely plan-specific — check your option agreement and plan document carefully:

• Many plans align the NQSO PTEP with the ISO window: 30–90 days after termination.
• Some plans give extended PTEPs for NQSOs — 1, 3, or even 10 years (until original expiration).
• If your NQSOs are in the money and the PTEP expires, they expire worthless with no recovery.

The key action: find your exact PTEP for each grant in your plan document before your last day.

Decision framework: should you exercise in the window?

The answer depends on four factors — and they interact.

1. Is the company public or private?

Public company: You know the value. Exercising and immediately selling (a disqualifying disposition for ISOs) gives you cash at the cost of ordinary income rates. Exercising and holding starts the LTCG clock — but you hold concentrated, single-stock risk. The analysis is straightforward: run the numbers.

Private company: Exercising costs cash (strike price) plus potential AMT (if ISO with a spread above the 409A), and you receive illiquid shares. You're betting the company will have a liquidity event. If it doesn't, or if the 409A drops, you may have paid out-of-pocket costs — including AMT — on value that evaporates.

2. How large is the spread?

A large spread on ISOs creates meaningful AMT exposure in the exercise year. The AMT preference item equals shares × (FMV at exercise − strike price). Compared against the 2026 AMT exemption ($90,100 for single filers; $140,200 for MFJ, phasing out starting at $500,000/$1,000,000 AMTI), large grants can trigger substantial AMT.³

At a private company with a high 409A, exercising as an ISO may trigger a six-figure AMT bill on shares you can't sell yet. In this case, it may be worth modeling whether forgoing ISO status and exercising as an NQSO (post-3-months) is financially comparable — or letting the options lapse if the company trajectory is uncertain.

3. Do you have the cash?

You need cash to cover: (a) the exercise cost — shares × strike price — and (b) the tax due. For ISOs at a public company you can cashless-exercise (sell immediately to cover costs). At a private company, you need your own liquidity. Early-stage companies sometimes run tender offers that create a secondary market; check if one is available or scheduled.

4. What's your conviction in the company?

If you're leaving because the company's prospects look poor, that conviction matters. Exercising pre-IPO options at a company you think will fail or be acquired below current 409A is throwing good money after bad — plus potential AMT on phantom gains. If your departure is unrelated to business trajectory and you believe in the long-term outcome, the calculus is different.

Special circumstances: disability, death, and retirement

The 3-month ISO rule has specific exceptions:

• Disability: Under IRC § 422(c)(6), if termination is due to disability (as defined in IRC § 22(e)(3)), the ISO exercise window extends to 12 months from the last day of employment.
• Death: ISOs held at death can be exercised by the estate or beneficiaries; the 3-month window runs from date of death (some plans allow longer). Check the plan document — estate administration timelines often conflict with option deadlines.
• Retirement provisions: Many plans — especially for senior employees who meet age and service criteria — extend the PTEP significantly or allow options to run until their original expiration. If you're in a "retirement eligible" category per your plan, your options may survive intact for years. Confirm this in writing with HR before your last day.

The 83(b) interaction for early-exercised shares

If you previously early-exercised unvested shares and filed an 83(b) election, termination doesn't immediately trigger tax on unvested shares. However, unvested shares are typically subject to company repurchase at exercise price when you leave. The repurchase gives back your original exercise cost — there's no gain, but no further upside either. Vested shares from a prior early exercise are yours to keep.

If you early-exercised but did not file an 83(b) election: each unvested tranche is still a potential income event as it vests. Leaving triggers a specific question — does your departure accelerate vesting (unlikely) or terminate it (common)? Read your agreements. A terminated unvested grant with no 83(b) filing typically generates no taxable income since you never received the shares.

QSBS: preserving the Section 1202 exclusion

If your company qualifies as a Qualified Small Business (C-corp, aggregate gross assets ≤ $75M at time of issuance for post-July 2025 stock, ≤ $50M for earlier stock), exercising options creates shares that may qualify for the Section 1202 exclusion — up to $15M per issuer (post-OBBBA) on stock issued after July 4, 2025.⁴

You cannot hold QSBS if you never exercise. Options are not QSBS — the stock acquired upon exercise is. If you let your options lapse without exercising, you lose any QSBS eligibility permanently. For companies where the QSBS math works — large potential exit, 5-year holding horizon, genuine IPO or acquisition probability — the post-OBBBA 100% exclusion at 5 years can be worth millions. Exercise before the PTEP expires to preserve the option.

Common mistakes in the exit window

• Assuming you have 90 days when your plan says 30. Always read your actual grant agreement. Default to the plan document, not common knowledge.
• Exercising all options the last week before expiration in a panic. Rushed exercises typically result in disqualifying dispositions (same-day sell) that wipe out any ISO benefit. Plan this before your last day — or in your first week afterward, not your last.
• Not separating grants. If you have ISOs from 2020, 2021, and 2023 at different strikes, each grant has its own timeline, AMT exposure, and qualifying-disposition clock. Treating them as a single block leads to suboptimal decisions.
• Ignoring the AMT credit. AMT paid on ISO exercises generates a credit recoverable in future years when regular tax exceeds AMT. If you're entering a lower-income year (which leaving a high-comp job often produces), you may be able to recover prior-year AMT credits quickly. This changes the calculus on exercising ISOs even with large spreads.
• Letting significant in-the-money options expire. If options are well in the money and you're within the PTEP, letting them expire is almost never the right answer. Even a same-day sale at ordinary income rates recovers real cash.

When does a specialist pay for itself?

The post-termination window is the highest-urgency scenario in stock option planning. The deadline is fixed; the decisions are irreversible; the stakes are often $500K–$5M+. A specialist who has modeled dozens of post-termination exercises can typically pay for themselves in a single hour of analysis — identifying the optimal exercise approach, timing across the PTEP, AMT positioning, and QSBS eligibility in a way that generic tax software won't catch.

For any option grant with intrinsic value above $250K, get a second opinion before your PTEP expires. The cost of acting suboptimally is typically a multiple of the advisory fee.

Sources

1. IRC § 422 — Incentive Stock Options. Section 422(a)(2): exercise must occur within 3 months of termination to retain ISO status. Section 422(c)(6): disability extends window to 12 months. Social Security wage base $184,500 for 2026 per SSA.
2. Tax Foundation — 2026 Long-Term Capital Gains Brackets. 20% LTCG rate applies above $533,400 single / $600,050 MFJ. 3.8% NIIT on net investment income above $200,000 single / $250,000 MFJ. Values verified April 2026.
3. IRS — 2026 AMT Inflation Adjustments. AMT exemption: $90,100 single / $140,200 MFJ. Phaseout begins $500,000 AMTI (single) / $1,000,000 AMTI (MFJ) at 50-cent rate per dollar (OBBBA change from prior 25-cent rate). Verified April 2026.
4. IRC § 1202 — QSBS Qualified Small Business Stock. OBBBA (July 2025): per-issuer gain exclusion raised to $15M (inflation-adjusted from 2027); tiered exclusion: 50% at 3 years, 75% at 4, 100% at 5 for stock issued after July 4, 2025. Gross asset threshold raised to $75M. Prior rules ($10M / 10x basis, 5-year full hold, $50M threshold) govern stock issued before that date.
5. IRS Publication 525 — Taxable and Nontaxable Income. Stock options section: ISO and NQSO taxation at exercise and sale, qualifying and disqualifying disposition rules, post-termination treatment.

Tax values verified against 2026 IRS guidance and OBBBA (July 2025). Post-termination option decisions are time-sensitive and irreversible — specialist review before the PTEP expires is strongly recommended.