The goals of this study were two-fold: (1) to test the hypothesis that retention of patients in naltrexone treatment could be improved by educating and preparing them for it while enrolled in a LAAM (l-alpha-acetylmethadol) detoxification program; and (2) to compare the safety and efficacy of 60 mg vs. 120 mg administered thrice weekly in a double-blind, sequential trial design. Patients were allowed a maximum of 365 days on naltrexone, and a maximum of four admissions; 119 patients received at least one dose of naltrexone. We found no clinically important differences between the two dosages, and retention in treatment was similar to that reported in earlier clinical trials. Slightly more than half the patients ever used heroin while receiving naltrexone, and only 9% of all urine tests were positive for opiates. Craving for heroin decreased dramatically by the end of the first week. We found no side-effect or toxicity due to naltrexone.