The impact of a system of primary pediatric care on emergency room use in a municipal hospital was measured by comparing that use in two randomly selected populations. The population that was offered participation in a primary care program consistently used the emergency room less than did the control group. Analysis of the pattern of utilization revealed that the differences were limited to patients who actually participated in the program, and to weekday use of the emergency room. Unusually heavy (greater than ten visits per year) use of the emergency room was virtually eliminated among participants in the primary care program.