Increasing numbers of severely mentally ill individuals are being treated in nonhospital, community-based settings and public concern about potential violence by these individuals has increased, often as a result of tragic, albeit uncommon events. The present study examines potential predictors of serious violence among persons with severe mental illness (SMI), with a specific focus on the joint effect of substance abuse and medication noncompliance. Subjects in the study are involuntarily admitted inpatients with SMI awaiting a period of court-ordered outpatient treatment, termed "involuntary outpatient commitment". During enrollment in a longitudinal outcome study of the effectiveness of OPC, 331 subjects and, whenever feasible, family members or other informants were interviewed. In addition, complementary data were gathered by review of involuntary commitment records and hospital records. Data collection included sociodemographic characteristics, illness history, clinical status, medication adherence, substance abuse and violent behavior during the 4 months preceding hospitalization. Descriptive and multivariable logistic regression procedures were used to examine the association between serious violent acts and a number of personal, social, and clinical characteristics. The combination of medication noncompliance and substance abuse was a significant predictor of serious violent acts in the community. Individuals who had problems with both alcohol and illicit drug abuse appear to be at greatest risk for violence. These results suggest that reducing violence risk among persons with SMI requires an aggressive approach to improving medication adherence in the context of integrated mental health and substance abuse treatment.