The occurrence of alcohol problems in opiate addicts on methadone maintenance presents a serious challenge for treatment. Dually addicted patients have a higher incidence of medical and psychiatric complications, consume large amounts of staff and treatment resources, and often exit treatment through administrative detoxification. Yet, little is known about differences in psychopathology between this group and other opioid addicts. This paper summarizes data for 166 male opioid addicts receiving methadone treatment. Alcohol problems were identified, and the relationship between dual addiction and psychopathology was explored. In the first phase of the study the MMPI and Brief MAST (Michigan Alcoholism Screening Test) were administered to 91 subjects. Comparison of Brief MAST scores with program counselors' judgments of the presence or absence of an alcohol problem showed a significant relationship. Cluster analysis of MMPI scores generated three groups. The clusters had significantly different mean MAST scores with the normal cluster having the least indication of alcohol problems, the neurotic cluster intermediate, and the schizoid cluster the greatest indication of problematic alcohol consumption. The second phase of the study with another 75 subjects replicated the relationship between cluster membership and MAST score. The Addiction Severity Index (ASI) was also administered to these subjects, and high correlations between Brief MAST and ASI alcohol problem measures were found. Within-treatment outcome data on a number of measures were then collected for 1 year. Problem drinking severity was negatively related to employment status and positively related to number of random urinalyses indicating illicit drug use. Implications of these findings for refining treatment strategies are discussed.