The hypothesis that competitive pressures encourage hospitals to accommodate patient and physician preferences for longer lengths of stay was tested. Seven hundred forty-seven nonfederal short-term hospitals were divided in terms of the number of neighboring hospitals within a 24-km radius, and this measure of hospital concentration and competition was measured against length of stay for ten surgical procedures, using 1982 data on 498454 patient discharges. Patient, physician, and hospital characteristics associated with length of stay were controlled for. Competition-related percentage increases in length of stay were identified for all procedures, including total hip replacement (14.8%), transurethral prostatectomy (13.9%), intestinal operations (14.0%), stomach operations (14.7%), hysterectomy (6.9%), cholecystectomy (9.1%), hernia repair (10.5%), appendectomy (8.4%), cardiac catheterization (22.9%), and coronary artery bypass graft surgery (21.2%). It was concluded that there is a strong association between the number of hospital competitors in the local market and the average length of stay in US hospitals.