Adverse health outcomes for uninsured patients have been attributed to their health status and to the quality of treatment received. A question about treatment that remains unexplored is whether physicians treating the uninsured are more likely to have characteristics indicative of lower quality than physicians treating insured patients. Using education, training, experience, and board certification to measure physician quality, we find that uninsured and Medicaid patients are treated by lower-quality physicians both because of the hospitals these patients attend and because of sorting within hospitals. The effects are statistically significant, but small.