Between the early 1980s and 2002, both the prevalence of obesity and the number of beneficiaries of the Social Security Disability Insurance program doubled. We test whether these trends are related; specifically, we test whether obesity causes disability and movement onto the disability rolls. We estimate models of instrumental variables using two nationally representative data sets: the Panel Survey of Income Dynamics and the National Longitudinal Survey of Youth, 1979 Cohort. The results are mixed but we find evidence that weight increases the probability of health-related work limitations and the probability of receiving disabilityrelated income. Our results suggest that the failure to treat obesity as endogenous leads to dramatic underestimates of the link between obesity and disability outcomes.