AIMS: Twelve-Step affiliation among adolescents is little understood. We examined 12-Step affiliation and its association with substance use outcomes 3 years post-treatment intake among adolescents seeking chemical dependency (CD) treatment in a private, managed-care health plan. We also examined the effects of social support and religious service attendance on the relationship. DESIGN: We analyzed data for 357 adolescents, aged 13-18, who entered treatment at four Kaiser Permanente Northern California CD programs between March 2000 and May 2002 and completed both baseline and 3-year follow-up interviews. MEASURES: Measures at follow-up included alcohol and drug use, 12-Step affiliation, social support and frequency of religious service attendance. FINDINGS: At 3 years, 68 adolescents (19%) reported attending any 12-Step meetings, and 49 (14%) reported involvement in at least one of seven 12-Step activities, in the previous 6 months. Multivariate logistic regression analyses indicated that after controlling individual and treatment factors, 12-Step attendance at 1 year was marginally significant, while 12-Step attendance at 3 years was associated with both alcohol and drug abstinence at 3 years [odds ratio (OR) 2.58, P < 0.05 and OR 2.53, P < 0.05, respectively]. Similarly, 12-Step activity involvement was associated significantly with 30-day alcohol and drug abstinence. There are possible mediating effects of social support and religious service attendance on the relationship between post-treatment 12-Step affiliation and 3-year outcomes. CONCLUSIONS: The findings suggest the importance of 12-Step affiliation in maintaining long-term recovery, and help to understand the mechanism through which it works among adolescents.