The National Treatment Outcome Research Study (NTORS) is a longitudinal, multisite, prospective cohort study that assesses changes in illicit drug use and other problems after treatment in national (UK) drug misuse treatment programmes. Clients were recruited from 54 residential and community-based drug treatment programmes throughout England. Four modalities were studied: inpatient drug dependence units, residential/rehabilitation programmes, methadone maintenance, and methadone reduction programmes. Data on substance use behaviours and physical and psychological health were collected by structured face-to-face interviews at intake, 1- and 2-year follow-up. Data are presented for 549 clients. A majority of clients achieved widespread improvements across a range of outcome measures after treatment in existing treatment services. For most outcomes, reductions in problem behaviours at the group level occurred within the first year and were maintained at 2 years. Considerable stability of outcomes at the individual level was also found. Abstinence from illicit drugs was substantially increased among clients from both residential and community programmes, and there were also substantial reductions in frequency of use of heroin, nonprescribed methadone, benzodiazepines, and crack cocaine. Injecting and shared use of injecting equipment was also reduced. Heavy drinking was common at intake and was not reduced at follow-up. Psychological and physical health problems were reduced on both groups at follow-up. These changes represent important clinical benefits to the individual clients, to their families, and to society.