Aims: To study mortality rate and causes of death among opioid addicts treated for self-poisoning or voluntary detoxification in Oslo 1980-81, and to compare the observed versus expected number of deaths. Design: A prospective cohort study of 185 opioid addicts either treated for self-poisoning (n=93, 1 year), voluntary detoxification (n=75, 2 years) or both (n=17), in all medical departments in Oslo in 1980/81. Setting: All deaths occurring by the end of 2000/2001 were identified from the Central Population Register. Causes of death were obtained from Statistics Norway. Participants: Median age was 24 years, range 16-41, 53 % were males.Measurements: Standard mortality ratios (SMRs) were computed, both for mortality in general and different causes of death in particular. Findings: During 20 years, 70 opiate addicts died (37.8 %), SMR 23.6 (18.7-29.9). The SMR remained high during the whole period, ranging from 32.4 in the first 5-year period, to 13.4 in the last period. There were no significant differences in SMR between self-poisonings and those admitted for voluntarily detoxification. The causes of death were accidents (11.4 %), suicide (7.1%), cancer (4.3%), cardiovascular disease (2.9%), violent deaths (2.9%) and other diseases (71.4%). Among the 50 deaths classified as other diseases, the category drug dependence caused the vast majority (37 deaths, 74.0%). The risk of death was significantly higher for all causes of death compared to the general population. Conclusion: The risk of death is high among opioid addicts, with a poor prognosis in a young patient group during a 20-year period.