In this study we compared the demographics of and reasons for visits in national samples of visits to retail clinics, primary care physicians (PCPs), and emergency departments (EDs). We found that retail clinics appear to be serving a patient population that is underserved by PCPs. Ten clinical problems such as sinusitis and immunizations encompass more than 90 percent of retail clinic visits. These same ten clinical problems make up 13 percent of adult PCP visits, 30 percent of pediatric PCP visits, and 12 percent of ED visits. Whether there will be a future shift of care from EDs or PCPs to retail clinics is unknown.