Emerging epidemics of HIV in Russia, India and China will largely determine the future course of the HIV pandemic. Injecting drug use has been a major source of new infections in these countries. The evolving role of injecting drug use and sexual transmission in driving these emerging epidemics is investigated using a mathematical model. HIV prevalence projections based on behavioural data for urban Russia result in a wide range of possible outcomes, reflecting uncertainty in estimates of adult sexual behaviour. Surveys of adult sexual behaviour in all the three countries are limited, and represent a research priority if the futures of these emerging epidemics are to be better understood. Analysis of behavioural correlates with adult HIV prevalence reveal the central role of unsafe sex in driving HIV prevalence, even among injecting drug users. However, needle sharing can also play a very significant role, particularly when the potential for heterosexual transmission is limited. These emerging epidemics are more likely to cross higher prevalence thresholds when there is extensive sexual mixing between sex workers and the general population, and to a certain extent between injecting drug users and non-users. Both types of sexual mixing have been documented in Russia, India and China. © 2003 Elsevier Science B.V. All rights reserved.