Information about automobile versus pedestrian or bicyclist collisions differ according to the source of collision ascertainment. Hospital records and police reports focus on different characteristics of a collision, which reflects differences in case finding as well as information regarding risk factors. Pedestrians and bicyclists 0-14 years of age who were involved in a motor vehicle collision in the city of Long Beach, CA, between January 1, 1992 and June 30, 1995, were included in the study. Police reports and medical records were used to identify cases. Police reports were found for 1,015 cases, and medical records identified 474 cases; 379 cases were common to both sources. A capture-recapture model was used to evaluate the degree of overlap between the two sources and to derive "ascertainment corrected" injury rates. The injury rate from combined sources was 333.5/100,000 children per year, the pedestrian injury rate was 210.6/100,000 children per year and the bicyclist injury rate was 122.9/100,000 children per year. The "ascertainment corrected" injury rate overall was 381.3/100,000 children per year, 233.0/100,000 for pedestrians and 153.9/100,000 children per year for bicyclists. Eighty percent of hospital-reported cases were captured in the police database, whereas only 37% of police-reported cases were captured by the hospital database. We found that hospital sources identified younger children, fewer bicyclists, more Asian and Hispanic children, and fewer African-American children when compared to police sources. For more comprehensive surveillance resulting in more accurate incidence rates and more complete information, it is better to use multiple sources of data.