We report a prospective study of medically treated unintentional injuries ascertained in an HMO population of children and adolescents 19 years of age and younger over a one-year period. The overall rate of medically treated injuries was 247 per 1,000, 147/1,000 for those treated in the clinic, and 100/1,000 for those treated in the emergency room (ER). Overall, 2.5 percent of patients were hospitalized. The highest rates of injury were for falls (60 per 1,000), recreational activities (57 per 1,000) and competitive sports (49 per 1,000). The site of care varied little by injury diagnosis or etiology, with the exception that sprains/strains, fractures, and injuries due to play and sports were more commonly treated in the clinic. The impact of trauma on activity was considerable, with 55.9 percent of injuries resulting in restricted activity, 10.6 percent in greater than or equal to 2 days of school missed, and 6.7 percent in greater than or equal to 2 days spent in bed.