In this review of published findings on human immunodeficiency virus (HIV) infection and risk factors among female sex workers, we summarize the results of seroprevalence studies in different countries and discuss the different patterns of transmission among such workers in various geographical regions. The highest rates of HIV infection occur in sub-Saharan Africa, where the widespread existence of sexually transmitted diseases may play an important role in sustaining transmission. In Europe and North America injecting drug use continues to be the major factor associated with HIV infection among female sex workers, while in Latin America and parts of Asia there is a more mixed pattern of heterosexual and parenteral transmission from injecting drug use. Reviewed also are studies of the risk factors associated with HIV infection among female sex workers, such as drug use, sexual behaviour, the presence of sexually transmitted diseases, and condom use; in addition, we comment on some studies of the clients of sex workers. Finally, we propose directions that future research in this area might take and discuss various interventions that need to be undertaken to reduce HIV transmission among female sex workers.