Efforts to understand the relationship between market structure and the quality of health services are complicated by non-random character of patients choices of where to recieve care. To address this problem, I construct an empirical model of helath outcomes for dialysis patients that accounts for the endogenous selection of which facility patients choose to recieve treatment from. The models estimates of facilities average quality are robust to both unobservable variation in condition severity and heterogeneous responses to different facilities treatment regims