The 2003 Medicare Prescription Drug, Improvement, and Modernization Act (MMA) approved tax-advantaged health savings accounts (HSAs) for certain high-deductible health insurance plans. We predict that MMA could lead to approximately 3.2 million HSA contracts among Americans ages 19-64 who are not students, not enrolled in public health insurance plans, and not eligible for group coverage as a dependent. We simulate the effect of several additional tax subsidies for HSAs. We predict that the Bush administration's refundable tax-credit proposal would double HSA take-up and reduce the number of uninsured people by 2.9 million, at an annual cost of $8.1 billion.