Data were analyzed from the nationwide Treatment Outcome Prospective Study (TOPS) to assess whether current and former methadone patients substitute other drugs for heroin. The sample comprised 513 heroin users who were admitted to methadone programs in 10 cities across the United States and followed for at least 1 year. Structured face-to-face interviews were administered at admission and at follow-up to assess use of six substances: cocaine, amphetamines, illegal methadone, tranquilizers, marijuana, and alcohol. The study found a decline in the use of all substances except alcohol. Patients who substantially reduced or eliminated their use of heroin during the follow-up year were more likely to decrease their use of other drugs than were patients who continued to use heroin on a weekly or more frequent basis. These findings suggest that methadone programs indirectly reduce patients' use of cocaine, amphetamines, illegal methadone, tranquilizers, and marijuana, insofar as they are successful in eliminating or decreasing heroin use. Similar reductions in drug use were found among patients who were not enrolled in methadone programs during the follow-up year. These findings do not support the commonly held belief that heroin addicts substitute other drugs for heroin.