Purchasers of medical care in the United States have focused attention on the gatekeeping role performed by primary care physicians as a means to control expenditures while assuring access to care. This research reports on the experience of a group of Medicaid beneficiaries in Missouri whose primary care providers agreed to become formal gatekeepers to virtually all medical services for their patients enrolled in the program. The results suggest that this relationship contributed to changes in health care utilization, including reductions in use of emergency rooms, specialists, and nonphysician providers and some increase in the likelihood of obtaining care from a primary care physician. There was, however, no evidence of significant cost reductions.