Those earning $100,000 to $150,000 per year and saved in both vehicles had average 401(k) balances of $226,800, but those saving only in their 401(k) had 23 percent less, or $174,200.

Of Fidelity’s HSA account holders, 23 percent were characterized as savers or those who spend only 10 percent or less of their annual contributions, electing to invest their remaining balances for health care costs in retirement; 33 percent are spenders, or those who spend 90 percent or more of their annual contributions on qualified medical expenses; and 44 percent are hybrids or those that spend their contributions but also leave year-end remaining balances.

Fidelity studied its HSA account holders who made continuous contributions to an HSA over the three years 2009 to 2011[v] and uncovered an increased average balance across all categories of 59 percent, from $3,200 to $5,100. But Savers saw a more than 100 percent increase in average balances, from $5,900 to $12,100. For Hybrids, average balances grew 79 percent, from $3,300 to $5,900.

“Our data show that over time, these accounts are becoming meaningful long-term savings vehicles for many Americans as they prepare for the escalating health care costs expected in retirement,” said Applegate.

Fidelity Investments is one of the world’s largest providers of financial services, with assets under administration of $3.7 trillion, including managed assets of $1.6 trillion, as of Aug. 31, 2012.