To estimate the impact of Medicare Part D on prescription drug coverage among elderly Medicare beneficiaries and to analyze the predictors of program enrollment (“take-up”) among those with no prior drug coverage.Multivariate analyses of data from the 2002, 2004, and 2006 waves of the Health and Retirement Study.Take-up of Part D among those without drug coverage in 2004 was high; about 50%-60% of this group had Part D coverage in 2006. Only 7% of senior citizens lacked drug coverage in 2006 compared with 24% in 2004. Demand for prescription drugs was the most important determinant of the decision to enroll in Part D among those with no prior coverage. Many of those who remained without coverage in 2006 reported that they do not use prescribed medicines, and the majority had relatively low out-of-pocket spending.For the most part, Medicare beneficiaries seem to have been able to make economically rational decisions about Part D enrollment despite the complexity of the program.