Competition and prospective payment have been widely used to control health care costs but may together provide incentives to selectively reduce expenditures on high-cost relative to low-cost patients. We use patient discharge and hospital financial data from California to examine the effects of competition on costs for high- and low-cost admissions in the 12 largest Diagnosis-Related Groups before and after the Medicare Prospective Payment System (PPS). We find that competition increased costs before PPS, but that this effect decreased afterward, especially inpatients with the highest costs. We conclude that competition and PPS selectively reduced spending among the most expensive patients and that careful assessment of these patients' outcomes is important.