"Federal tax rules for HSAs were liberalized in 2003, making them very attractive to well-heeled taxpayers. Commercial banks such as Bank of America and Mellon Bank, seeing the opportunity to collect management fees on the accounts, jumped into the business.

"Every bank wants to increase its share of HSAs," said John Casillas, director of the Medical Banking Project, a Franklin, Tenn., organization that helps medical administrators develop financial service systems.

"There's fees for managing the account, transaction fees, fees for investing the funds," Casillas said. "You're going to see many billions of dollars moving from premium payments to professionally managed investment funds under HSA rules. Some people think that banks are going to threaten health plans by replacing them in the marketplace.""