This study represents the first controlled trial of methadone maintenance and 45-day methadone detoxification in Thailand. We randomly assigned 240 male heroin addicts with at least six prior detoxification treatment episodes to 45-day detoxification or to methadone maintenance treatment conditions. We found that methadone maintenance clients were more likely to complete 45 days of treatment (p < .00001), were less likely to have used heroin during treatment (p < .0002), and were less likely to have used heroin on the 45th day of treatment (p < .000007). These findings are consistent with the results of other studies in the United States, and we conclude that methadone maintenance could be a significant adjunct to AIDS prevention efforts for intravenous drug users in Thailand.