In a three-year field of methadone maintenance programs in New York City (U.S.A.) Philadelphia, and Baltimore, treatment was found to be effective in reducing Iv drug use and needle sharing among most heroin addicts. Of 388 patients who remained in treatment for one year or more, 71 percent had ceased IV use. Conversely, 82 percent of patients who left treatment relapsed rapidly to IV drug use. Marked differences in the effectiveness of various programs were observed: current Iv use varied from less than 10 percent to over 57 percent of patients in particular treatment programs. This differential effectiveness was related both to length of patient''s stay and the quality of treatment provided. HIV seropositivity among high-risk drug users is related to frequency of injections and needle-sharing contacts. Effective methadone treatment can stop these practices, but a widespread impact on the AIDS epidemic will require improvement of treatment in many programs.