Although group therapy is widely used for individuals with substance use disorders (SUDs), randomized clinical trials (RCTs) comparing the same treatment in a group versus individual format are rare. This paper presents the results of a RCT comparing guided self-change (GSC) treatment, a cognitive-behavioral motivational intervention, conducted in a group versus individual format with 212 alcohol abusers and 52 drug abusers who voluntarily sought outpatient treatment. Treatment outcomes demonstrated significant and large reductions in clients' alcohol and drug use during treatment and at the 12-month follow-up, with no significant differences between the group and individual therapy conditions. A therapist time ratio analysis found that it took 41.4% less therapist time to treat clients using the group versus the individual format. Participants' end-of-treatment group cohesion scores characterized the groups as having high engagement, low levels of interpersonal conflict, and low avoidance of group work, all desirable group characteristics. These findings suggest that the GSC treatment model was effectively integrated into a brief group treatment format. Health care cost containment compels further evaluations of the efficacy of group treatments for SUDs.