Just 4 percent of all accounts, or about one million, have at least part of the money invested, according to the H.S.A. services firm Devenir.
There were about 26 million H.S.A.s holding about $61.7 billion as of midyear, according to Devenir.
Devenir offers a search tool, HSASearch.com, to help shoppers compare accounts. But visit each plan’s website before making a selection to make sure you have the latest details, like current interest rates.