A new analysis finds that a diligent saver who leaves the money untouched for decades can accumulate $1 million. But not everyone with an H.S.A. can afford to leave the money untapped.

It’s possible to amass $1 million in special health savings accounts to use in retirement, a new analysis finds, with several big caveats.

You have to start young, contribute the maximum each year and leave the money untouched for decades instead of spending it on medical needs.

Health savings accounts, known as H.S.A.s, let people set aside pretax money for health and medical care.

To open an H.S.A., you must have a specific type of health plan with a high deductible — an amount you must cover out of pocket before insurance pays. The money can be saved or invested to grow tax-free, and is tax-free when withdrawn and spent on eligible care or products. (The federal government does not tax the

Keep reading this article on The New York Times Your Money.