Several case studies indicate that clozapine use is associated with reductions in the use of nicotine, alcohol, or illicit drugs. Although not designed to assess clozapine, this study explored a posteriori the effects of clozapine on alcohol and drug use disorders among schizophrenia patients. Among 151 patients with schizophrenia or schizoaffective disorder and co-occurring substance use disorder who were studied in a dual-disorder treatment program, 36 received clozapine during the study for standard clinical indications. All participants were assessed prospectively at baseline and every 6 months over 3 years for psychiatric symptoms and substance use. Alcohol-abusing patients taking clozapine experienced significant reductions in severity of alcohol abuse and days of alcohol use while on clozapine. For example, they averaged 54.1 drinking days during 6-month intervals while off clozapine and 12.5 drinking days while on clozapine. They also improved more than patients who did not receive clozapine. At the end of the study, 79.0 percent of the patients on clozapine were in remission from alcohol use disorder for 6 months or longer, while only 33.7 percent of those not taking clozapine were remitted. Findings related to other drugs in relation to clozapine were also positive but less clear because of the small number of patients with drug use disorders. This study was limited by the naturalistic design and the lack of prospective, standardized measures of clozapine use. The use of clozapine by patients with co-occurring substance disorders deserves further study in randomized clinical trials.