The authors report the findings of a longitudinal study testing the hypothesis that substance use leads to subsequent violence in the community. Subjects were 103 patients with a Structured Clinical Interview for DSM-III-R diagnosis of schizophrenia or schizoaffective disorder who were seen in an outpatient clinic for the treatment of schizophrenia. Data on substance use and violent behavior were collected by review of medical records. Results indicated that use of drugs and alcohol was associated with increased odds of concurrent and future violent behavior when compared with persons with schizophrenia and no substance use. Odds of violence were particularly elevated for individuals having a pattern of polysubstance use involving illicit substances.