Among 118 patients with DSM-III-R diagnoses of schizophrenia or schizoaffective disorder who received treatment with clozapine, 29 patients had an antecedent or current history of substance abuse. Substance abusers and nonabusers showed similar improvements on measures of psychopathology and psychosocial functioning after 6 months of clozapine therapy. The implications of this finding are discussed.