Dually diagnosed patients with chemical dependency and a comorbid psychiatric disorder typically show poor compliance with aftercare treatment, which may result in costly and pervasive individual and societal problems. In this study, the authors investigated the effect of adding motivational interviewing in a group format to standard treatment for dually diagnosed psychiatric inpatients. The patients (n = 101) all received standard care and in addition were assigned to either group motivational interviewing (GMI) or a therapist attention activity control group (TAAC). Of patients who attended aftercare and who used alcohol or drugs, those who participated in GMI attended significantly more aftercare treatment sessions, consumed less alcohol, and engaged in less binge drinking at follow-up compared with those in TAAC. Differences between conditions in the overall percentage of participants who achieved complete abstinence or who attended aftercare treatment were not significant, possibly because of a lack of power. These results provide preliminary evidence for the efficacy of GMI when added at the outset to an inpatient program.