OBJECTIVE: To assess factors associated with enrollment in a Medicare advantage (MA) plan versus Medicare fee-for-service plan in 2000-2004 by Medicare-eligible veterans. We also assessed whether these factors differed between disability-eligible veterans and age-eligible veterans. METHODS: Medicare claims data, VA administrative data, and 2000 census data were constructed in a retrospective cohort study of 20,581 age-eligible veterans and 7541 disability-eligible veterans. MA enrollment in 2000-2004 was estimated in a logistic regression in a pooled sample of age-eligible and disability-eligible veterans that controlled for demographic, socioeconomic, and disease risk factors. Separate logistic regressions also were estimated for age-eligible and disability-eligible veterans. RESULTS: Minority veterans and veterans with lower disease risk scores were more likely to be enrolled in an MA plan in 2000-2004 than white veterans or veterans with higher risk scores. Age-eligible veterans were more likely to be enrolled if aged 75 or older, female, able to receive free VA care, or not enrolled in Medicaid. Disability-eligible veterans were more likely to be enrolled if they were married or elderly. CONCLUSIONS: Medicare Advantage plans appeared to benefit from favorable selection of Medicare-eligible veterans.