Aims: To assess the effectiveness of a motivational interview among hospitalized psychiatric patients with comorbid substance use disorder in reducing alcohol and other drug (AOD) use. Design: Subjects were assigned randomly to receive an individual motivational interview (n=79) or a self-help booklet (control condition; n=81). Setting: Subjects were volunteers recruited from a major public psychiatric hospital. Participants: Subjects met abuse or dependence criteria on the structured clinical interview for diagnosis (SCID) for alcohol, cannabis or amphetamine or they reported hazardous use during the last month of one or more of these drug types on the opiate treatment index (OTI). Intervention: Either one 30–45-minute motivational interview or brief advice. Measurements: The SCID and OTI were the main measures. Findings: There was a modest short-term effect of the motivational interview on an aggregate index of alcohol and other drug use (polydrug use on the OTI). Cannabis use remained high among the sample over the 12-month follow-up period. Conclusion: Although motivational interviewing appears feasible among in-patients in psychiatric hospital with comorbid substance use disorders, more extensive interventions are recommended, continuing on an out-patient basis, particularly for cannabis use.