In a study designed to investigate the pattern of substance use disorders among a group of chronic mentally ill patients in Toronto, 102 patients completed the Structured Clinical Interview for DSM-111-R and a modified substance-use-disorder module of the Diagnostic Interview Schedule. Forty percent of the sample met criteria for substance use disorders, and 49 percent for personality disorder. Among patients with personality disorder, all those with a personality disorder in cluster B (that is, with antisocial, borderline, histrionic, or narcissistic personality disorder) had a substance use disorder, while the majority of patients in cluster A and cluster C were not substance abusers. in the overall sample, the group with substance use disorders was significantly younger than the group without. In contrast to findings of previous studies, women met criteria for substance use disorders as often as men did.