The effect of motivational interviewing on outpatient treatment adherence among psychiatric and dually diagnosed inpatients was investigated. Subjects were 121 psychiatric inpatients, 93 (77%) of whom had concomitant substance abuse/dependence disorders, who were randomly assigned to: a) standard treatment (ST), including pharmacotherapy, individual and group psychotherapy, activities therapy, milieu treatment, and discharge planning; or b) ST plus motivational interviewing (ST+MI), which involved 15 minutes of feedback on the results of a motivational assessment early in the hospitalization, and a 1-hour motivational interview just before discharge. Interviewers utilized motivational techniques described in Miller and Rollnick (1991), such as reflective listening, discussion of treatment obstacles, and elicitation of motivational statements. Results indicated that the proportion of patients who attended their first outpatient appointment was significantly higher for the ST+MI group (47%) than for the ST group (21%; χ2 = 8.87, df = 1, p < .01) overall, and for dually diagnosed patients (42% for ST+MI vs. 16% for ST only; χ2 = 7.68, df = 1, p < .01). Therefore, brief motivational interventions show promise in improving outpatient treatment adherence among psychiatric and dually diagnosed patients.