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We present a geographical analysis of the first 22 months of the Department of Defense's human immunodeficiency virus (HIV) screening program for military applicants. This analysis concerns four HIV epidemic loci in the United States: New York City; Miami, Florida: Houston, Texas; and San Francisco, California. The cartographic analysis, using a novel smoothing algorithm to stabilize county-based rates, highlights the diffusion of the HIV epidemic from areas with relatively high prevalence to areas with relatively low prevalence of antibody to HIV. While there are particularities specific to each region, the diffusion patterns are remarkably consistent: areas closer to an endemic focus experience a slight decline over time, while more distant areas increase. The diffusion process depicted in map sequences demonstrates expansion diffusion in all four areas, while some provide a clear indication of a mixture of expansion and hierarchical processes. A previous analysis of these data for a fifteen-month period had concluded there was no evidence of a geographic trend in the data. Those conclusions were based on all ages pooled and used multi-state geographic units, making it unlikely that trends in prevalence, which varied by proximity to HIV epicenters, would have been detected. In view of the temporal crossover between the endemic and nonendemic areas' prevalence rates observed in Florida, Texas and California, any spatial analysis of these data using statewide summary rates will obscure the spatial-temporal trends. These data provide compelling evidence for the existence of sub-state level epidemics growing spatially and temporally into areas some miles from the original epicenters. We hope these results will encourage other public health personnel to undertake detailed surveillance of the AIDS and HIV infection epidemics at a unit of analysis small enough to permit quantifying spatial diffusion of this epidemic. [ABSTRACT FROM AUTHOR]