The trend towards giving consumers choice about their health plans has invited research on how good they actually are at making these decisions. The introduction of Medicare Part D is an important example. Initial plan choices in this market were generally far from optimal. In this paper, we focus on plan choice in the years after initial enrollment. Due to changes in plan supply, consumer health status, and prescription drug needs, consumers’ optimal plans change over time. However, in Medicare Part D only about 10% of consumers switch plans every year, and on average, plan choices worsen for those who do not switch. We develop a two-stage panel data model of plan choice whose stages correspond to two separate reasons for inertia: inattention and switching costs. The model allows for unobserved heterogeneity that is correlated across the two decision stages. We estimate the model using administrative data on Medicare Part D claims from 2007 to 2010. We find that consumers are more likely to pay attention to plan choice if overspending in the last year is more salient and if their old plan gets worse, for instance due to premium increases. Moreover, conditional on attention there are significant switching costs. Separating the two stages of the switching decision is thus important when designing interventions that improve consumers’ plan choice.