Much has been learned about how to prevent HIV infection among psychoactive drug users in the last 25 years, but, worldwide, the problems of drug-use-related HIV transmission have increased during this time. We consider the need for additional research on four aspects of drug use-related HIV transmission: (1) why evidence-based effective prevention programmes have not been implemented, (2) HIV infection among ethnic minority drug users, (3) relationships--or lack of relationships--between individual risk behaviour and HIV infection and (4) reducing drug use-related sexual transmission of HIV and other sexually transmitted infections (STIs). These topics were selected because we see them as critical for reducing HIV transmission among drug users (topics 1 and 4), reducing health disparities among racial and ethnic groups of drug users (topic 2), and understanding HIV epidemiology and evaluating prevention programmes for drug users (topic 3).