The efficacy of Naltrexone in preventing reabuse of heroin among heroin addicts in Israel was studied in a double-blind, controlled design. Naltrexone (or placebo) treatment was given as part of a general treatment plan that continued for 12 weeks. Thirty-two addicts who successfully completed a detoxification program and met research criteria, were included in the study. Fifty milligrams of Naltrexone were taken orally three times a week (25 mg twice a week for the first 2 weeks). The follow-up procedure included an interview, urine tests, and screening for possible adverse effects. In addition, social and psychological parameters were evaluated. Fewer heroin-positive urine tests were found the Naltrexone group than in the placebo group. Throughout the entire study, the number of drug-free patients in the Naltrexone group was higher than in the placebo group. The Naltrexone group showed a significant improvement in most psychological parameters as compared with the placebo group. No differences were found in compliance or ratio of adverse effects between the Naltrexone and placebo groups. The concept "heroin abuse load" based on daily heroin consumption and duration of addiction enabled us to predict which addicts would complete the treatment program. The results suggest that heroin addicts in Israel may benefit from treatment with Naltrexone.