From March 1989 to February 1991, at "Telefono in Aiuto" (USL RM/10), Naltrexone has been used in 271 heroin addict patients. Naltrexone, a pure opiate antagonist, nullifies subjective heroin effects, preventing the setting up of tolerance and physical dependence. By administration for one year to drug-free addicts, it proved to be useful in relapse prevention. Naltrexone seems to be safe and without side-effects. Its effectiveness grows if its use is matched with psychological help.