This study, conducted in 2005 to 2007, presents results that are based on a proscriptive cohort design. The sample consisted of 769 residents in 22 drug user treatment programs who stayed in treatment for at least 30 days to one year; 510 former residents (66%) from 21 programs (95%) were interviewed again at a 6-month post-treatment follow-up assessment. A majority of the participants were male, lived with family or relatives, had completed only primary school, and had a full-time or a part-time job prior to entering treatment. The participating therapeutic community (TC) programs were a mixture of volunteer, compulsory-probation, and prison-based programs. In-person interview data and urine testing showed that the self-reported drug use prevalence rates are reliable. The results show large positive treatment effects on 30-day and 6-month illegal drug use and small to medium effects on the severity of alcohol use and related problems. A multilevel regression analysis suggests that residents' reduced stigma, adaptation of the TC model, and frequency of alcohol and drug use-related consequences partially predict treatment success. Study limitations and policy implications are discussed.