SSA approved about 2.2 million Medicare beneficiaries for the low-income subsidy as of March 2007, despite barriers it faced in identifying the eligible population and soliciting applications; however, measuring the success of SSA’s outreach efforts is difficult because there are no reliable data on the size of the eligible population. In 2005, SSA mailed 18.6 million subsidy applications to Medicare beneficiaries who were potentially eligible for the subsidy. SSA knew that this mailing was an overestimate, but took this approach to ensure that all who were eligible would be contacted. SSA had hoped to more specifically identify the eligible population using IRS tax data, but current law restricts the use of taxpayer data unless an individual has already applied for the subsidy. Further, SSA conducted a campaign of about 76,000 events held nationwide to educate people about the subsidy and how to apply for it. Since the initial campaign ended, however, SSA has not developed specific performance goals and measures to assess the progress of its continuing outreach efforts. SSA’s efforts to solicit applications were hindered by beneficiaries’ confusion about the difference between the subsidy and the Medicare Part D prescription drug plan, and the reluctance of some individuals to share personal financial information, among other factors. While the early subsidy participation rate compares favorably to those of some other low-income programs, the lack of reliable data on the size of the eligible population means that the extent to which SSA has signed up the eligible population for the benefit is unknown.