Alcohol and cocaine abuse result in unsatisfactory treatment outcomes for heroin and illicit opioid addicts engaged in Methadone Maintenance Treatment Programs (MMTPs). This study aims to clarify the impact of MMT, which focuses on cessation of opioid abuse and diminishing psychopathology to acceptable levels (stabilization) on alcohol and cocaine abuse. Of specific interest was whether reduction of polysubstance abuse and associated psychopathological complications diminished illicit opioid abuse and/or increased retention in treatment. Changes in cocaine and alcohol use that occurred in 53 heroin addicts who had been stabilized were monitored. A control group was composed of patients terminated from treatment due to noncompliance with treatment recommendations, poor attendance, or failure to have opioid abuse stabilized, within a year. The association of psychiatric severity with alcohol and cocaine abuse in these methadone maintained patients was assessed. Cessation of illicit opioid abuse and retention in treatment are positively correlated with decrease in alcohol and cocaine abuse and the absence of the psychosocial complications associated with such abuse.