This study describes the incidence of fatal and nonfatal injuries occurring in 87,022 Massachusetts children and adolescents during a one-year period. A surveillance system for injuries at 23 hospitals captured 93 per cent of all discharges for ages 0-19 in the 14 communities under study. Sample data were collected on emergency room visits, hospital admissions, and deaths for all but a few causes of unintentional injuries. The overall incidence was 2,239 per 10,000. The true incidence rates are probably higher than those reported. The ratio of emergency room visits to admissions to deaths was 1,300 to 45 to 1. Injury rates varied considerably by age, sex, cause, and level of severity. Age-specific injury rates were lowest for infants and elementary school age children and highest for toddlers and adolescents. The overall ratio of male to female injury rates was 1.66 to 1. Injuries from falls, sports, and cutting and piercing instruments had a high incidence and low severity. Injuries from motor vehicles, burns, and drownings had lower incidence, but greater severity. Results provide evidence that both morbidity and mortality must be considered when determining priorities for injury prevention. Current prevention efforts must be expanded to target injuries of higher incidence and within the adolescent population.