The abuse of heroin and other opiates by intravenous injection is identified as the major risk for the spread of infection by the human immunodeficiency virus (HIV) and of acquired immunodeficiency syndrome (AIDS) to the population at large. From 1980 to 1992, 2,712 opiate-addicted intravenous drug users with severe complications and behavioural problems were treated at the Institute on Addictions, Belgrade; 2,090 of them were from the Belgrade area and 622 from other urban areas. All of them had used heroin by intravenous injection during a certain period of their addiction careers. A majority of the patients had started using heroin by the age of 20, and begun treatment after six or more years of addiction. An informal survey of heroin-addicted intravenous drug users newly admitted for treatment showed that every respondent knew from 10 to 20 other heroin users who had not sought treatment. It was estimated that a majority of intravenous drug users may not have been known to the authorities. Of 551 intravenous-opiate-addicted patients from the Belgrade area tested between 1987 and 1992, 43.7 per cent were HIV-seropositive, or 47.9 per cent of HIV-seropositive males and 32.9 per cent of females, while for the same period, of 366 tested patients from other urban areas, 4.6 per cent were HIV-seropositive, 5.2 per cent of them males and 1.8 per cent females. The distribution by sex of the intravenous-opiate-addicted patients indicated that the percentage of females who started using heroin earlier in their lives and the percentage of those who sought treatment late--after six or more years of addiction--were higher than the corresponding percentages of males. Also, female patients tended to become infected with HIV earlier in their lives than male patients.