[Excerpt] he number of individuals participating in HSA-eligible health plans and HSAs increased significantly between 2004 and 2007; however, in all years, many HSA-eligible plan enrollees did not open an HSA. The number of individuals covered by HSA-eligible plans increased significantly between September 2004 and January 2007 from about 438,000 to approximately 4.5 million, according to industry estimates. Despite the growth, these plans represented a small share of individuals with private health coverage about 2 percent in 2006. The number of tax filers reporting HSA activity also increased, nearly tripling between 2004 and 2005, from about 120,000 to about 355,000. Industry estimates suggest continued growth in HSA participation in 2006 and 2007. Despite the growth in HSA participation, nationally representative survey estimates from 2005, 2006, and 2007 found that more than 40 percent of HSA-eligible health plan enrollees did not open an HSA. Tax filers who reported HSA activity in 2005 had higher incomes on average than other tax filers. Among tax filers between the ages of 19 and 64, the average AGI for filers reporting HSA activity was about $139,000 compared with about $57,000 for all other filers. The income differences existed across all age groups. The total value of all HSA contributions reported to IRS in 2005 was about twice that of withdrawals$754 million compared with $366 million. Among all filers reporting HSA activity in 2005, average contributions were about $2,100, compared to average withdrawals of about $1,000. Survey estimates of the contributions employers made to employees' HSAs in 2007 varied. One employer survey reported average contributions for single coverage of $626 among large employers, while another employer survey reported average contributions for single coverage of $806 among small and large employers. More than a third of surveyed employers that offered HSA-eligible plans made no HSA contributions.