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The Drug Abuse Treatment Outcome Study (DATOS) is the 3rd in a series of national multisite studies of community-based treatment sponsored by the National Institute on Drug Abuse. The major goal of this prospective cohort study of adult clients entering treatment from 1991 to 1993 is to evaluate treatment effectiveness. The study included 10,010 admissions from 96 programs in 11 cities. Interviews were conducted at admission and during treatment, and 2,966 selected participants completed a 12-month follow-up interview. This article describes the methodological aspects of the study and provides an overview of program and client samples. Data collection procedures and instrumentation are described, and the analytical approach used to attain the research objectives is presented. Future plans for a longer term follow-up are also described, along with the potential contributions of DATOS findings to treatment policy.