Although reported rates of human immunodeficiency virus (HIV) infection and related risk behaviours among drug injectors vary considerably throughout the world, and comparison of findings is often hampered by methodological variability among centres. In 1989 the World Health Organization initiated a comparative study of current drug-injecting behaviour and HIV infection using a standardized methodology. Centres were asked to recruit at least half of their samples outside drug treatment settings in order to achieve as representative a group of injectors as possible. Respondents were interviewed using a questionnaire designed by an international working group, and they were asked to provide blood and/or saliva specimens for voluntary testing. Data from 13 centres (Athens, Bangkok, Berlin, Glasgow, London, Madrid, Naples, New York, Rome, Rio de Janeiro, Santos, Sydney and Toronto) are reported here. A total of 6,390 injectors were recruited to the study from October 1989 to March 1992, with sample sizes ranging from 85 at Santos (Brazil) to 1,300 in New York. Weekly or daily sharing of injecting equipment was reported by less than a quarter of injectors in all centres. A high proportion of those sharing made some effort to clean equipment before use, although not always by efficient methods. In all centres, the majority of respondents were sexually active; however, rates of unprotected sexual intercourse were high, particularly between regular sexual partners. The overall HIV prevalence rate was 22 per cent, ranging from 0 per cent in Athens to 60 per cent in Santos. Caution should be exercised in postulating a link between HIV prevalence and current risk behaviour, particularly since injectors appear to behave in similar ways across a diverse range of study locations. This is the first report on an international collaborative study for which a large number of injectors were successfully recruited from a variety of settings. The wealth of data now available provides a greater understanding of the social epidemiology of drug injecting, which is essential for the implementation and evaluation of campaigns and interventions designed to limit the spread of HIV infection.