Aim. We examined the efficacy of naltrexone (an opioid antagonist) for alcohol dependence in a sample of alcohol-dependent men. Design. A 12-week randomized placebo-controlled clinical trial. Setting. The outpatient clinic of a combined war veteran and general teaching hospital in Melbourne, Australia. Participants. Male alcohol-dependent subjects recruited from the community and from veteran groups. Intervention. Alcohol-dependent subjects were treated with 50 mg of naltrexone or placebo daily for 12 weeks. Both treatment groups attended a weekly education support group. Subjects were assessed weekly. Measurements. Primary study outcomes were the maintenance of abstinence and relapse to drinking. Findings. Fifty-five subjects were randomized to naltrexone and 56 to placebo. Forty subjects did not complete 12 weeks of therapy (17 naltrexone, 23 placebo). In the intention-to-treat sample ( N = 111) fewer naltrexone treated subjects relapsed ( p = 0.001). Among patients who completed the 12-week trial, naltrexone reduced the consumption of alcohol. Naltrexone was well tolerated and there were few adverse experiences. Conclusions. These findings demonstrate that naltrexone is effective in preventing relapse to drinking in the setting of limited psychosocial treatment. Further studies should examine the duration of treatment needed to maintain the effect long term.