To extend the study of the epidemiology of interpersonal violence into the area of nonfatal intentional injury, a retrospective study was undertaken of patients admitted to a major city hospital for intentional injury during a single year (1984-1985). This study focused on interpersonal assaultive injury, excluding sexual assault, child abuse, and self-inflicted injuries. Relevant injuries made up three-quarters of all admitted intentional injuries and represented a total of 671 patients (4.5% of all hospital admissions). The intentionally injured were compared to the catchment area population and to the remaining admitted hospital population, by age, sex, and race. Medical records of intentionally injured adolescents (n = 133) were reviewed and compared to a sample of adolescent homicide victims regarding the circumstance of the event and the relationship of victim to assailant. Relevant findings are that: (1) in the area served by this hospital, male adolescents are at relatively high risk for nonfatal intentional injury; (2) among adolescents, the majority of these intentional injuries are the result of interpersonal conflicts between acquaintances, paralleling the etiology of homicide: (3) a prevalence of missing data in the medical records of older male adolescents is symptomatic of the lack of attention that has been paid to understanding and preventing intentional injury among adolescents. It is concluded that: (1) more area-specific and hospital-based studies of intentional injury are needed as a guide to such preventive efforts; and (2) medical personnel providing acute care to victims of intentional injury are an important resource for this research and prevention effort.