OBJECTIVE: The authors outline a minimal set of outcome indicators to assess the effects of specialized treatment for people with severe mental illness and substance use disorders and report on use of these indicators in a longitudinal study of such treatments. METHODS: A total of 147 clients with dual disorders participated in a controlled clinical trial of three interventions--behavioral skills training, case management, and 12-step recovery--in a county mental health program. The clients were assessed every six months over a two-year period using multidimensional self-report and observer-rated outcome measures encompassing psychosocial functioning, psychiatric and substance abuse symptoms, and service utilization. RESULTS: Client self-reports showed changes in psychosocial functioning, especially increased functioning in residential stability and work, and reductions in alcohol and drug symptoms and usage. Data on service utilization showed decreased use of acute and subacute mental health services and increased use of outpatient and case management services over time. Ratings by trained observers of psychiatric symptoms and psychosocial functioning improved dramatically. CONCLUSIONS: A minimal set of outcome indicators for clinical trials and demonstrations of interventions for clients with dual disorders should include client self-reports of social adjustment, life satisfaction, psychiatric and substance abuse symptoms, and current substance use; interviewers' ratings of psychosocial functioning and psychiatric symptoms; data on utilization of mental health treatment and support services; and data on clients' personal income, use of medical services, and contact with the criminal justice system.