The evaluation was conducted over an 18-month period for a 133-patient cohort as they entered or remained in methadone maintenance for narcotic addiction. During the study period (July 1992-December 1993), the 80 patients who were already in treatment at the beginning of the study and the 53 patients who entered treatment at the Department of Veterans Affairs in the East Orange methadone program (New Jersey) were evaluated. At the beginning of the study, all patients already in treatment were assessed by chart review for the prevalence of cocaine abuse at the time of admission into treatment. These patients were followed for the duration of the study. All new patients were enrolled in the study as they entered treatment. Over the 18 months of the study, all patients were assessed objectively and subjectively for cocaine and opioid abuse at least monthly. Overall retention during the study was 85 percent, with significantly (p < .05) higher daily methadone doses in those patients still in treatment at the end of the study. Predictably, illicit opioid use was dramatically reduced, to 10 percent as measured by urine toxicology in the last month of treatment. Moreover, significantly more patients stopped regular cocaine abuse (69 percent) than started using cocaine. Thus, with effective methadone maintenance that used adequate dosages, the majority of patients remained in treatment and reduced cocaine abuse as well as illicit opioid use, with implications for public health by reducing the spread of infectious diseases, including hepatitis B, C, D, and human immunodeficiency virus. 7 figures and 42 references