For high-price drugs, Medicare Part D beneficiaries who do not receive a low-income subsidy must pay a percentage of the drug's price for each medication fill. Without that subsidy, which lowers out-of-pocket spending, beneficiaries typically pay hundreds or thousands of dollars for a single fill. We estimated the proportion of Part D beneficiaries in fee-for-service Medicare, with and without a subsidy, who do not initiate treatment (that is, do not fill a new prescription) with high-price Part D drugs newly prescribed for four conditions. Examining 17,076 new prescriptions issued between 2012 and 2018 for Part D beneficiaries from eleven geographically diverse health systems, we found that beneficiaries receiving subsidies were nearly twice as likely to obtain the prescribed drug within ninety days as those without subsidies. Among beneficiaries without subsidies, we observed noninitiation for 30 percent of prescriptions written for anticancer drugs, 22 percent for hepatitis C treatments, and more than 50 percent for disease-modifying therapies for either immune system disorders or hypercholesterolemia. Our findings support current legislative efforts to increase the accessibility of high-price medications by reducing out-of-pocket expenses under Medicare Part D, particularly for beneficiaries without low-income subsidies.