The law that created Medicare's prescription drug benefit, Medicare Part D, also established extra help for low-income seniors in the form of a subsidy. This study, the first in-depth analysis of Part D enrollment among Medicare beneficiaries without prior drug coverage, finds that 63 percent of all eligible seniors and 69 percent of low-income beneficiaries were enrolled in Part D in 2006. However, only 29 percent of low-income beneficiaries were enrolled in the subsidy program, leaving millions without coverage. Many reported that premiums were too costly, enrollment too difficult, and information too hard to obtain for enrollment. Additionally, provisions of the recently enacted Patient Protection and Affordable Care Act may have the perverse impact of reducing enrollment in Part D for certain beneficiaries. Our findings emphasize the need to expand eligibility and improve policies to foster enrollment.