The human immunodeficiency virus (HIV) pandemic continues to grow at an alarming rate, with a further 5 million new infections in 2003. Some 3.5 million of these were in sub-Saharan Africa, where approximately 70% of the world's HIV-positive population resides. In contrast, the spread of HIV in high-income countries has slowed since its discovery in the 1980s, and in regions such as Western Europe prevalence has decreased. Here, we employ coalescent methods to compare the epidemic growth rates of two subtypes of HIV-1 with differing epidemiological profiles: subtype C, which is dominant in sub-Saharan Africa and associated with heterosexual transmission, and subtype B, the main cause of AIDS in Western Europe and North America, and which was primarily transmitted through homosexual sex and injecting drug use. We show that although both subtypes emerged at approximately the same time ( approximately 1960), they have widely differing patterns of exponential population growth. At its current growth rate the epidemic of subtype C in sub-Saharan Africa is doubling every 2.4 years, which is approximately half the rate observed during the early stages of the subtype B epidemic in Western Europe and North America. However, the subtype C growth rate is still 5-10 times greater than that estimated for the blood-borne hepatitis C virus, supporting the hypothesis that sexual transmission has been primarily responsible for the HIV epidemic in sub-Saharan Africa.