In common with most countries, little is know about the geographic distribution of human immunodeficiency virus (HIV) in South Africa. Variations in HIV infection of persons 15-24 years of age were modeled and mapped using generalized linear spatial models and Bayesian prediction based on data from a national HIV household survey conducted in 2003 and comprising 11,904 youth from approximately 700 enumeration areas that were randomly selected from the national census. The maps show considerable variation in HIV prevalence within provinces. The lowest levels were found in inland rural areas of the Western Cape, and the highest in northwestern parts of KwaZulu Natal, southern Mpumalanga, and eastern Free State. Prevalence of HIV was associated with ethnicity, urban status, and unemployment. Detailed maps of HIV prevalence can be effectively used in guiding and focusing intervention programs to areas of particular need.