This study investigates criminality among 331 opiate abusers admitted to Stockholm's methadone maintenance programme (SMMP) between 1988 and 1992, and a comparison group of 1483 untreated opiate abusers. Information on arrests, criminal convictions, and intravenous drug abuse was obtained from official records. For both genders the annual rate of convictions decreased from 2.2 convictions per year during the 4 years prior to the first treatment, to 0.5 convictions during treatment, compared to 2.0 convictions for the comparison group. There was an even greater decrease in the rate of arrests for patients on methadone treatment. The decline was observed for both genders and in both HIV-positive and HIV-negative patients. Rates of convictions among patients who had more than one treatment period were clearly reduced during each treatment period, and while the rate increased after they were expelled from treatment it remained at a lower level than during the 4 years prior to treatment. Thus, the methadone treatment is shown to have a profound positive effect on arrests and convictions, not only for patients remaining in treatment but also for those patients who were expelled from treatment involuntarily.