An ISA is one of the cleanest tax shelters available to a UK resident. To the IRS it is not a shelter at all. There is no US recognition of the wrapper, so income and gains inside it are simply taxable to you as a US person in the year they arise.
That alone would be an annoyance. The real problem is what the ISA holds — and the regime the US applies to it.
Why the funds inside are the real issue
Unit trusts, OEICs, investment trusts and most non-US ETFs are almost all Passive Foreign Investment Companies under US rules — the test is mechanical and pooled funds pass it by construction. Default PFIC treatment spreads a gain across your holding period, taxes each slice at that year's top ordinary rate, and adds an interest charge on the 'late' tax.
Held long enough, the effective rate on a PFIC gain routinely exceeds 50% — on an investment that would have been lightly taxed on either side of the Atlantic alone. The reporting burden, Form 8621 per fund per year, is disproportionate to the sums involved even when no tax is due.
The elections, and their timing
There are elections that improve matters. Mark-to-market is the most commonly useful for listed holdings — gains are taxed annually as ordinary income, which is worse than capital gains treatment but far better than the default regime. A QEF election is cleaner still but requires fund-published data that UK retail funds rarely provide.
Both generally have to be made in the right year — usually the first year of holding or the first year of US personhood. That is why this is worth looking at before a position has been held and grown for a decade, when every year of delay adds to the eventual interest charge.
What a clean portfolio looks like
None of this means a US person in the UK cannot invest. Shares in an ordinary operating company are not PFICs — though a foreign company that is itself mostly cash or investments can be one, so holding companies deserve a second look. US-domiciled ETFs that hold HMRC 'reporting fund' status are treated rationally by both systems, and they exist for every major index. Cash ISAs still do their UK job — HMRC leaves the interest alone — but the US taxes it with no UK tax paid to credit against, so the wrapper saves nothing on that side.
For an existing PFIC portfolio, the honest arithmetic is often a planned purge: realise the positions, pay the historic charge once, and rebuild in clean funds. The wrong move is doing nothing while the interest clock runs. We model the cost of leaving against staying before recommending either.
What actually makes something a PFIC
The test is mechanical and it looks at the company, not at the wrapper around it. A foreign corporation is a passive foreign investment company if 75% or more of its gross income is passive, or if 50% or more of its assets produce passive income. Either test alone is enough.
A pooled fund meets both by construction, because holding investments is the entire point of it. That is why the answer for a UK unit trust, OEIC or non-US ETF is almost always yes, regardless of what the fund invests in or how conservatively it is run.
The rules can also look through one company to another beneath it, so a feeder structure does not solve anything. Our PFIC checker walks the ordinary cases, and whether a specific fund is caught covers the checks worth doing before you buy.
The three regimes, side by side
Three treatments exist, and which one applies decides almost everything about the cost. The default is the one that applies when nothing is elected, which is the position most people discover themselves to be in.
| Treatment | How it taxes | What it needs |
|---|---|---|
| Excess distribution (default) | Gains spread back over the holding period at top ordinary rates, plus an interest charge | Nothing - it applies by default |
| Qualified electing fund | Your share of the fund's income each year, as it arises | An annual information statement most UK funds do not produce |
| Mark to market | The annual change in value, as ordinary income | Marketable stock, and an election made in time |
| Inside a pension | Generally outside this treatment altogether | A recognised scheme |
| Reporting | One Form 8621 per fund per year | Records per fund, not per account |
Lifetime ISAs, Junior ISAs and the rest of the family
Every ISA in the range shares the same flaw for an American holder: the wrapper is a UK construct and the US does not see it. What changes between them is what sits inside and who owns it.
A Lifetime ISA adds a government bonus that has no American equivalent, and the US taxes the income and gains inside regardless of it. A Junior ISA belongs to the child, so the analysis follows the child rather than the parent - our guide to Junior ISAs and US tax covers what that means for a dual-citizen child.
A cash version of any of them avoids the fund problem entirely. The interest is taxable in the United States and no British tax was paid to credit against it, which is irritating rather than punitive, and far better than the alternative.
Rebuilding a portfolio, step by step
The order matters more than the speed. Selling everything on discovering the problem can crystallise the worst version of the treatment in a single tax year.
- List every fund you hold, with its domicile, purchase date and cost.
- Establish which holdings are caught and how long each has been held.
- Check whether any election is still available, since some close with the passage of time.
- Model a phased exit across tax years against simply holding, and price both.
- Direct new contributions into structures that do not create the problem.
- Fill pension capacity first, since a recognised scheme generally sits outside this treatment.
- Keep the number of separate funds low, because the reporting cost scales with it.
An illustrative example
Take an illustrative example: an American in Edinburgh who has paid into a stocks and shares ISA for eleven years, holding three UK-domiciled index funds. In the UK the account is exemplary and entirely tax-free.
On the American side she holds three passive foreign investment companies, each needing its own annual form, with eleven years of holding period behind the eventual gain. Nothing about her investing was careless; the structure simply was never chosen with the second system in mind.
Her options are real but none of them is free. Modelling a phased sale against continuing to hold, and doing it before she needs the money, is what turns an unpleasant surprise into a decision.
Common mistakes with ISAs and US tax
The first is assuming the ISA is simply invisible to the IRS. It is visible, it is taxable, and the account itself may also need reporting on the balance-driven forms.
The second is assuming no distribution means no consequence. The treatment attaches to the holding and to the eventual disposal, not only to what was paid out along the way.
The third is selling in haste. A rushed exit can produce the worst available outcome in one year, where a phased one across two or three tax years would not.
