The comparative effectiveness of 12-step and cognitive-behavioral (C-B) models of substance abuse treatment was examined among 3,018 patients from 15 programs at U.S. Department of Veterans Affairs Medical Centers. Across program types, participants showed significant improvements in functioning from treatment admission to a 1-year follow-up. Although 12-step patients were somewhat more likely to be abstinent at the 1-year follow-up, 12-step, C-B, and combined 12-Step-C-B treatment programs were equally effective in reducing substance use and improving most other areas of functioning. The finding of equal effectiveness was consistent over several treatment subgroups: Patients attending the "purest" 12-step and C-B treatment programs, and patients who had received the "full dose" of treatment. Also, patients with only substance abuse diagnoses, those with concomitant psychiatric diagnoses, and patients who were mandated to treatment showed similar improvement at the 1-year follow-up, regardless of type of treatment received. These data provide important new evidence supporting the effectiveness of 12-step treatment.