INTRODUCTION: Previous research has reported switching from traditional Medicare (TM) to Medicare Advantage (MA) plans increased from 2006 to 2011 at the aggregate level, and switching from MA plans to TM also increased. However, little is known about switching behavior among individuals with specific chronic diseases. OBJECTIVE: To examine disease-specific switching patterns between TM and MA to understand the impact on MA plans. METHODS: Using the 2006 to 2012 Medicare Current Beneficiary Survey, we examined disease-specific switching rates between TM and MA and disease-specific ratios of mean baseline total Medicare expenditures of beneficiaries remaining in the same plan (stayers) vs those switching to another plan (switchers), respectively. We focused on beneficiaries with 1 or more of 10 incident diagnoses. RESULTS: Beneficiaries with a new diagnosis of Alzheimer disease and related dementias, hypertension, and psychiatric disorders had relatively high rates of switching into MA plans and low rates of switching out of MA plans. Among those with new diagnoses of psychiatric disorders and diabetes, more costly beneficiaries (those with higher costs) switched into MA plans. For cancer, more costly beneficiaries remained in MA plans. CONCLUSION: Together, these results suggest that MA plans may have not only higher caseloads but also a more costly case mix of beneficiaries with certain diseases than historically was the case. Our findings can help inform MA plans to understand their beneficiaries' disease burden and prepare for provision of relevant services.