This study was designed to evaluate the effects of a child passenger safety law on pediatric motor vehicle trauma, as seen from the perspective of a hospital emergency room setting. The data were obtained from an ongoing monitoring system consisting of nine hospital emergency rooms in Orange County, California, and the county coroner's office. All children under the age of 15 years evaluated in the monitored emergency rooms after involvement in a motor vehicle crash were included. The years 1981 and 1982 constituted the prelaw period; 1983 and 1984, the postlaw period. Those children 4 years of age or older, namely, those who were not covered by the child safety law, were the control population. The major findings for children less than 4 years of age were that (1) restraint use increased from 26% in the prelaw period to 50% in the postlaw period; (2) a significant decrease in the number of injured was documented; (3) head injuries decreased by 17%; and (4) hospital emergency room utilization did not decrease.