Naltrexone and placebo as adjuvant treatment of opioid dependence were compared in a double-blind, controlled clinical trial in 50 heroin addicts. The overall efficacy was assessed by the degree of treatment acceptance, percentage of relapse in heroin consumption, presence of side effects, and overall retention on naltrexone. A total of 50 patients of both sexes, aged from 18 to 30 years, who fulfilled DSM-III-R criteria for opioid dependence were included in the study. All patients completed detoxification with clonidine on an inpatient basis for 2 weeks and subsequently, on an out-patient basis, received oral naltrexone (350 mg per week) for a month. At the beginning of the second month patients were randomly allocated to treatment with naltrexone (28 patients) or placebo (22 patients) until a 6-month treatment period in a double-blind fashion had been completed. During the study period (1 year) all patients followed the same therapeutic schedule. Patients in both groups were comparable in terms of socio-demographic data and toxicological history. The efficacy of naltrexone was not superior to that of placebo as there were no significant differences in acceptance of treatment, retention rates, opioid and other drug consumption, drug compliance or side effects.