Within the Great Lakes region of East Africa previous research on the emergence of HIV/AIDS has focused primarily on Uganda's Central and Tanzania's Kagera Regions, and the locally dominant ethnic groups, the Ganda and the Haya. The patterns of sexual behaviour which facilitated the rapid spread of HIV have typically been associated with changes which occurred during the 1970s, a decade characterised by declining state services, growing violence and black marketeering, and worsening sexual vulnerability. This article argues that in order to explain the emergence of HIV in the Great Lakes as a whole, it is necessary to construct a series of local narratives of sexual behavioural change over a longer time period. Only then can the specific character of the region's various sub-epidemics, as well as their commonalities, be fully understood. This article traces the evolution of sexual attitudes and behaviour between 1900 and 1980 in a society that neighbours Buganda and Buhaya, Ankole in western Uganda. It explains how Ankole's relatively austere sexual culture gradually came to replicate that of Buganda between the 1930s and 1980, arguing that it was the 1960s that saw the most important changes in sexual behaviour and attitudes.