We estimated age-specific and sex-specific incidence rates of intentional injuries (assaults or suicide attempts) occurring between 1979 and 1982 in a population of 87,022 Massachusetts children and adolescents under 20 years of age in 14 communities with populations of 100,000 or less. The average annual incidence of intentional injuries treated at a hospital was estimated to be 76.2 per 10,000 person-years. Overall, 1 in 130 children was treated each year for an intentional injury. More than 85 percent of the injuries resulted from assaults, such as fights, rape, and child battering; 11.4 percent were self-inflicted. Intentional injuries were most common among adolescents. Each year, 1 in 42 teenage boys was treated for an assault-related injury, and 1 in 303 teenage girls was seen for a suicide attempt. Repeated episodes of intentional injury were identified in 4.3 percent of the children. In this population, intentional injuries accounted for 3.4 percent of all injuries but 9.8 percent of hospital admissions and 15.7 percent of deaths from injury. The rate of intentional injury was directly correlated with both the degree of urbanization and the poverty level of the community of residence. We conclude that intentional injuries are relatively common in this population and that attempts to prevent them must be directed to the children who are at greatest risk.