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Objective: To measure HIV prevalence at health-district level in the Western Cape (WC) and to compare these findings with those of the National HIV Antenatal Surveys (NHASs). This investigation aimed to estimate the degree of heterogeneity of HIV prevalence within the province in order to inform the design of appropriate and targeted HIV interventions. Method: Annual cross-sectional, unlinked district HIV antenatal surveys were implemented in all 25 health districts of the WC for the years 2001 - 2004, concurrently with the NHAS. A stratified proportional sample was drawn for each district, involving all 344 antenatal clinics in the province, and the anonymous screening method as described by the World Health Organization (WHO) was applied.Results: The NHAS revealed a significant increase in HIV prevalence in the WC from 8.6% (95% confidence interval (CI): 5.6 - 11.6) in 2001 to 15.4% (95% CI: 12.5 - 18.2) in 2004. The district-level HIV surveys showed wide variation in HIV prevalence across the health districts, which increased progressively during this period (a range of 0.6 - 22% for the year 2001 increased to 1 - 33% in 2004). Spatial analysis of HIV prevalence by health district for this period also revealed progressive spatial growth of the sub-epidemics, with the highest prevalence observed in districts located in the Cape metropole region. Conclusions: These concurrent surveys highlight the fact that examining a provincial estimate of HIV prevalence alone has the potential to mask epicentres within the province. This underscores the importance of expanding the surveillance systems to detect heterogeneity sub-provincially, in order to link with local-level planning and resource allocation. South African Medical Journal Vol. 96(6) 2006: 538-543