When you borrow against your portfolio and use the proceeds for investment purposes, the interest you pay may be deductible against your net investment income. It's one of the more valuable and underused deductions available to investors who regularly use leverage.

The catch is documentation. The IRS requires what's called interest tracing — a clear record showing where borrowed funds went. If you draw $500K from a portfolio line and wire it into a capital call for a private equity fund, that interest expense can be deducted against investment income. If you draw the same $500K and deposit it into a checking account that also funds personal expenses, the tracing breaks down and the deduction may not hold up.

This gets complicated quickly for investors with multiple credit lines used for different purposes. A single portfolio line might fund a capital call in January, cover a tax payment in April, and bridge a real estate closing in August. Each use has a different tax treatment, and the IRS expects you to be able to show which dollars went where.

The deduction is limited to net investment income, which means you can only deduct investment interest up to the amount of dividends, interest, and short-term capital gains you recognize in the same year. Unused amounts carry forward, but the benefit is delayed. This is an itemized deduction, so it only applies at the federal level if you're itemizing — but for investors carrying this level of borrowing, that's almost always the case.

What makes this worth getting right: for someone in the top federal and California brackets paying 5–6% interest on a $3M portfolio line used for investment purposes, the deduction can be worth $75K–$90K per year in tax savings. That's material, but only if the documentation supports it.

The simplest approach is to keep investment borrowing in a dedicated account that isn't used for anything else. One line, one purpose, clean tracing. For investors who use credit more flexibly, working with a CPA to establish a tracing log at the time of each draw — not at year-end — is the difference between a defensible deduction and one that gets unwound in an audit.

For educational purposes only. Not tax, legal, or investment advice.