Concurrent dependence on cocaine and alcohol is common among patients seeking addiction treatment. This study was undertaken to explore the effectiveness of naltrexone (150 mg) as a potential treatment for patients who are alcohol and cocaine dependent. Of 15 subjects enrolled in the 12-week, open medication trial, 7 subjects did not complete the study. Relapse to clinically significant drinking occurred in 7 subjects (47%). There was a reduction in the average daily amount of alcohol consumed from pretreatment to treatment (p < .001) and the percentage of days engaged in drinking behavior (p < .001). Similarly, there was a reduction in the average weekly amount spent on cocaine from pretreatment to treatment (p = .001) and the percentage of days using cocaine (p < .001). This preliminary study suggests that naltrexone (150 mg) may be tolerable in patients dependent upon alcohol and cocaine and may be effective in reducing both cocaine and alcohol use. The results of this study provide a rationale for a double-blind placebo-controlled study of the efficacy of naltrexone in this difficult to treat but prevalent population.