National observational studies indicate that infants who are not restrained in Child Safety Seats (CSSs) are riding onlap. This study was undertaken to determine the conditions under which children travel onlap. The extent to which injuries would be reduced if these children were restrained in CSSs was also examined. Data were taken from a multi-hospital monitoring system for pediatric occupant injuries and the Coroner's office in a single urban county (1980-1989). Only children under one year of age were included. Information was obtained from medical records and parent interviews. One hundred ten children were traveling on-lap. This did not appear to result from overcrowding; 88% were riding in vehicles with five or less occupants; 83% were in the front seat; 58% sustained injury; 22% were hospitalized; and 15% sustained intracranial injury. A 26% reduction in overall injury, a 75% reduction in hospitalization, and a 69% reduction in intracranial injury was projected for those riding on-lap, had they been restrained in CSSs. Applying national rates of on-lap travel (16.8%) to NHTSA estimates of the number of infants injured in crashes each year, indicates that approximately 2,218 of these children would be on the lap of another passenger. Substantial savings in terms of hospitalization, intracranial injury, deaths, and costs, can be realized if children traveling on the lap of another occupant were in CSSs. Parents frequently place the child on the lap in order to attend to the child's needs, feed the child, or because they feel the child is more secure in this position. Parent education as well as strict enforcement of CSS laws must be implemented. Parents must be convinced that this practice of travel in vehicles is not protective, and, in fact, poses a significant risk for injury.