OBJECTIVE: This study examined the relationship between violence and substance abuse among patients with chronic mental illness living in the community. METHODS: All referrals over a one-year period to an urban assertive community treatment team were evaluated systematically with a standardized intake protocol. Thirty-seven patients with a history of violence in the community were compared with 27 patients without such a history on a variety of clinical and demographic variables. RESULTS: More than half of the patients (58 percent) had a history of violence in the community. The only significant differences between those with a history of violence and those without involved alcohol or drug use. The single best predictor of violence was the onset of alcohol or drug abuse in late childhood or early adolescence. CONCLUSIONS: In this sample, very early onset of substance abuse among people who developed mental illness was associated with the greatest risk of community violence. Thus at least some of the causal determinants of violence in this sample may precede the onset of adult mental illness.