Hospitalization accounts for a large portion of the expenditures for child health care, and differences in the rate of hospitalization may produce important variations in the cost of that care. We studied the rates of hospitalization in Boston, Rochester (N.Y.), and New Haven (Conn.) in 1982. We assigned the risk of hospitalization in Rochester a score of 1.00. Boston children were hospitalized at more than twice the rate of Rochester children for most medical diagnostic categories (relative risk, 2.65; 95 percent confidence interval, 2.53 to 2.78), and the rate for the New Haven group was intermediate (relative risk, 1.80; 95 percent confidence interval, 1.68 to 1.93). Rates of inpatient surgery differed less (Boston relative risk, 1.12; New Haven relative risk, 0.93). The relative risks of hospitalization (as compared with Rochester children) for Boston and New Haven children, respectively, were 3.8 and 2.3 for asthma, 6.1 and 2.9 for toxic ingestions, and 2.6 and 2.7 for head injuries. Fractures of the femur, appendicitis, and bacterial meningitis (conditions uniformly treated in the hospital) had similar rates of hospitalization across the three cities, but the relative risk of hospitalization for aseptic meningitis was 3.7 in Boston. The rates of hospitalization of children in all three communities were below the national averages in 1982. Although this study does not define the reasons for the variation in rates of hospitalization, it is possible that they were related in part to differences in socioeconomic status or access to primary care. The implications of these data for the cost and quality of pediatric care therefore remain to be determined.