This paper describes the political origins of the National Treatment Outcomes Research Study (NTORS) and the outputs and impacts of the study. NTORS was designed to meet the request of the Health Secretary and of a Government Task Force for evidence about the effectiveness of the national addiction treatment services. NTORS was a prospective cohort study which investigated outcomes over a 5-year period of drug users admitted to four major treatment modalities: in-patient treatment, residential rehabilitation, methadone reduction and methadone maintenance programmes. The study investigated treatments delivered under day-to-day operating conditions. Outcomes showed substantial reductions in illicit drug use and reduced injecting risk behaviours. These changes were accompanied by improved psychological and physical health and by reductions in criminal behaviour. However, not all outcomes were so positive. There was a continuing mortality rate in the cohort of about 1% per year, and many clients continued to drink heavily throughout the 5-year follow-up. NTORS findings informed and influenced UK addiction treatment policy both at the time and subsequently. The findings were influential in supporting an immediate increase in funding for treatment, and Government Ministers have repeatedly cited NTORS as evidence of the effectiveness of addiction treatment. One finding that received political attention was that of the cost savings provided by treatment through reductions in crime. This important finding led to an unanticipated consequence of NTORS; namely, the greater focus on crime reduction that has increasingly been promoted as a political and social priority for drug misuse treatment.