Since the inception of the Medicare End-Stage Renal Disease (ESRD) Program in 1972, the number of persons qualifying for this benefit has increased every year, even when controlling for growth of the US population. At the same time, the profile of the typical dialysis patient has changed dramatically over the years, with new enrollees tending to be older and sicker. This study attempts to explain case mix through an economic model of a dialysis provider who certifies eligibility of Medicare ESRD beneficiaries and manages their care. The model was tested using administrative data from the Medicare ESRD program and other sources. Results demonstrated that dialysis patient case mix is influenced by local area demand for medical care and factors such as competition or payment levels that might affect a provider's supply decisions.