Data were collected on three forms developed for the project: the Initial interview, the Clinical Data Form, and the Follow-up Interview. The Initial Interview, conducted at the time of enrollment, obtained personal background information, drug use history, and other information. The Clinical Data Form, completed 1 year after admission, collected uniform data from the clinical record. Methadone dose, urinalysis results, months on methadone, and other data were entered on this form. The Follow-up Interview, completed as soon as possible after the first anniversary of admission, obtained information about months in specified activities during the year following admission. In addition, the questions previously asked in the initial interview about activities during the 30 days preceding admission were repeated for the 30 days preceding the first anniversary of admission. At the end of the first year, 52 percent were on methadone, and 48 percent were off methadone. Among subjects on methadone, days of intravenous drug use, crime, and needle-sharing decreased markedly from the month preceding admission to the month preceding the first anniversary of admission. Among subjects off methadone, days of these activities also decreased, but the decreases appeared in large part to be an effect of increased days of incarceration. Among those on methadone, days of productive activity increased markedly. Subjects with more years of intravenous drug use were more likely to be on methadone at the end of the year, and subjects under compulsory supervision were less likely to be on methadone. The findings confirm previous reports of decreased illicit opioid use, decreased crime, and decreased needle risk for infection with the HIV among opioid users who remain on methadone. 3 tables and 23 references