Substance abuse was examined in a group of 187 chronic mentally ill patients living in the community. According to ratings by aftercare clinicians, approximately one-third of the sample ahused alcohol, street drugs, or both during the six months before evaluation. Patients who were dually diagnosed with substance abuse and mental illness differed from those with mental illness alone in several respects: they were younger and more often male; were less able to manage their lives in the community in terms of maintaining regular meals, adequate finances, stable housing, and regular activities; showed greater hostility, suicidality, and speech disorganization; and had poorer medication compliance. In addition, they were nearly twice as likely to be rehospitalized during one-year follow-up. Substance abuse appeared to add the problems of disruptive, disinhibited, noncompliant behaviors to chronic mental illness.