OBJECTIVE: The study assessed the prevalence of severe aggressive behavior and conduct disorder in a population of runaway and homeless adolescents and examined relationships between aggression, conduct disorder, other problem behaviors, and background characteristics. METHODS: A total of 219 runaway and homeless youths recruited through a urban drop-in center were surveyed using the Adolescent Health Survey, a questionnaire about background and mental health experiences, and the revised version of the Diagnostic Interview Schedule for Children. RESULTS: More than half of study participants met criteria for conduct disorder, and 62 percent reported a history of severe aggressive behavior. Although these constructs were related to each other, a third of the subjects met criteria for only one. Childhood sexual abuse was associated with conduct disorder, while living in a home where drugs were used was associated with aggression. Severe aggressive behavior was associated with other problem behaviors, including attempted suicide, behavior that precipitated residential psychiatric treatment, pregnancy, arrests, and convictions. CONCLUSIONS: The assessment and systematic treatment of conduct disorder and aggression among runaway and homeless youths is urgently needed to reduce the effects of the disorder and associated problem behaviors.