Data from 12 different European countries show a rapid increase in HIV antibody positivity among drug users or a high degree of contamination already reached wherever studies have been made. Until 31 December 1986, 698 (18%) of AIDS cases were among drug users, of which 600 (15%) of AIDS cases were solely drug users, and 98 (3%) were in addition homosexual or bisexual. A further increase is expected. Because of the epidemiological importance for transmission to the heterosexual population, this problem has become a focus of attention. Drug abusing prostitutes constitute a major source of infection for the heterosexual population and newborns. The increase in the number of AIDS cases in 1986 among male drug abusers was 98 - that is up 61% compared to previous years; among women, the increase was 56%. The 3 main approaches to solution of this problem, i.e. interdiction of the drug trade, availability of sterile needles, and an education program have not proven as successful as anticipated. Relevant indications of the progress of infection in society can only be obtained by systematic observation of conversion rates in differential subgroups, i.e. drug abusers, newly incarcerated drug abusers, male and female prostitutes who use drugs, and individuals newly reporting for treatment. Separation of HIV antibody positives and negatives in therapeutic communities which are not drug free is recommended for epidemiological purposes in view of the developments to date. Nor should forced segregation of the infected from noninfected be dismissed out of hand.