DATOS is a comprehensive, longitudinal, nationwide, multi-site study of drug abuse treatment effectiveness. It is the third in a series of evaluations in the United States and included client admissions to treatment programs from 1991 to 1993. Over 10,000 individuals were interviewed in 96 programs from 11 American cities at entry into treatment and while they remained in treatment or aftercare. Participants were interviewed at entry to treatment and during treatment, then at 12 months, 36 months, and 5 years after termination from their index treatment program. The 5-year follow-up sample included 1,393 of the same individuals in the 1-year follow-up sample.