The most pressing issue in health care delivery today is inflationary cost increases. The gatekeeping role of primary care physicians, particularly family physicians, may lower health care costs through a more judicious use of specialty referrals, expensive tests, and hospitalization. The study of such an impact is most readily carried out in the practice setting of health maintenance organizations (HMOs), where there is a defined patient population. Incomplete data and lack of sensitive indicators of the gatekeeping effect are limitations of this preliminary study. The results show, however, that the internal organization of an HMO does not influence hospital and ambulatory care utilization rates, with the exception that HMOs staffed by a group of salaried physicians (staff HMOs) reported higher ambulatory care utilization. No significant differences were demonstrated in hospital or ambulatory care utilization rates among the HMOs using more primary care physicians or family physicians than others. The results indicate that ambulatory care utilization rates are proportional to the number of physicians per 1,000 members. The results also suggest that there may be an inverse relationship between hospital utilization rates and the number of primary care physicians, especially if they are family physicians. Further studies need more specific indicators to evaluate the effect of the gatekeeping role in health care delivery.