Fueled by growth in the large-employer market, overall enrollment in health savings accounts linked to high-deductible health plans increased by more than 13% to nearly 19.7 million this year, according to an annual survey released Wednesday.

In the large-employer market — defined in the survey by Washington-based trade group America's Health Insurance Plans as employers with at least 51 employees — 15.4 million people were enrolled in HSA-linked plans at the start of 2015, a 20.3% increase over 2014.

Enrollment in the small-group market dipped and edged up slightly in the individual market. However, AHIP believes the “census understates the number of individual purchasers choosing HDHP plans with health savings accounts because some market-specific data is unavailable for certain states,” the survey said.

In the case of the small-group market, reported enrollment was 2.3 million, down 2.5 million in 2014. Reported enrollment in the individual market was 2,042,520 this year this year, up from 1,974,895 in 2014.

“HSA plans are a valuable and affordable coverage option for millions of consumers and their families,” AHIP President and CEO Marilyn Tavenner said in a statement. “These plans offer families the flexibility to save for future medical expenses and make informed decisions about their health care needs,” she added.

HSAs, authorized under a 2003 law that added a prescription drug benefit to the Medicare program, became available on Jan. 1, 2004. Enrollment has risen steadily since then.

AHIP said 1 million people were enrolled in HSAs in 2005, 3.2 million in 2006, 4.5 million in 2007, 6.1 million in 2008, 8 million in 2009, 10 million in 2010, 11.4 million in 2011, 13.5 million in 2012, 15.5 million in 2013, 17.4 million in 2014 and 19.7 million in 2015.

States with the highest number of HSA enrollees at the start of this year were Texas at 1.5 million, Illinois at 1.3 million and Ohio at 842,000. States with the lowest number of enrollees at the start of this year were Hawaii at 652, Rhode Island, 14,267, and Wyoming 20,044.

The key factor driving HSA growth is that premiums for high-deductible health plans, which the law requires be linked to HSAs, tend to be much lower than for more traditional health plans, according to numerous surveys.

For example, a survey by Mercer L.L.C. found that the cost of medical coverage through a consumer-driven health plan linked to an HSA averaged $8,732 per employee among employers with at least 500 employees in 2014. That's about 21% less than the average of $11,121 per employee for medical coverage through a preferred provider organization.