This paper presents an evaluation of clinical characteristics of opiate addicts who also have a current or past history of alcoholism diagnosed according to Research Diagnostic Criteria. Based on data derived from a survey of 533 treated opiate addicts, the following findings were obtained. First, alcoholism in treated opiate addicts was found to be a common problem occurring at rates far exceeding those in the surrounding community in all demographic groups. Second, regarding the sequence of alcoholism and opiate addiction, most alcoholic addicts in this sample had alcohol problems before developing a drug use disorder or seeking treatment for opiate addiction. Third, entrance into treatment did not appear to lead to either increased or decreased alcohol abuse, and alcohol abuse while in treatment usually took place in those who had a history of alcohol problems. Fourth, when compared with nonalcoholic addicts, alcoholic addicts had fewer assets and more liabilities including a more disruptive childhood history, heavier history of legal problems and polydrug abuse, more severe problems with social functioning, and higher rates of psychiatric disorders. Fifth, despite these liabilities follow-up findings 6 months after seeking treatment indicate that alcoholism was of little prognostic significance in relationship to treatment outcome.