Elderly Americans, especially those with multiple chronic conditions, face difficulties paying for prescriptions, which results in worse adherence to and discontinuation of therapy, called cost-related medication nonadherence. Medicare Part D, implemented in January 2006, was supposed to address issues of affordability for prescriptions. We investigated whether the gains in medication affordability attributable to Part D persisted during the six years that followed its implementation. Overall, we found continued incremental improvements in medication affordability in the period 2007-09 that eroded during the period 2009-11. Among elderly beneficiaries with four or more chronic conditions, we observed an increase in the prevalence of cost-related nonadherence from 14.4 percent in 2009 to 17.0 percent in 2011, reversing previous downward trends. Similarly, the prevalence among the sickest elderly of forgoing basic needs to purchase medicines decreased from 8.7 percent in 2007 to 6.8 percent in 2009 but rose to 10.2 percent in 2011. Our findings highlight the need for targeted policy efforts to alleviate the persistent burden of drug treatment costs on this vulnerable population.