The results of the treatment evaluation research based on the Drug Abuse Reporting Program (DARP) are summarized and discussed. The DARP is a data system containing almost 44,000 admissions during 1969 to 1973 to 52 treatment programs located throughout the United States and in Puerto Rico. The current report focuses on the findings of a number of interlocking posttreatment follow-up studies based on data for three independent samples representing admissions to this system. These data include a total of 4,627 interviews conducted 5 to 7 years after admission to the DARP, and an average of over 4 years after termination from treatment. The overall findings indicate that treatment in methadone maintenance, therapeutic communities, and outpatient drug-free programs was effective in improving posttreatment performance with respect to drug use, criminality, and productive activities. Significantly poorer outcomes were reported for outpatient detoxification programs and a group of comparison (intake only) clients.