Since routine testing began, 677,463 members of the Reserve Components of the US Army have been tested for antibody to human immunodeficiency virus (HIV). Of these, 1063 were positive, for a crude prevalence of 1.57/1000 tested. Prevalence varied greatly among different groups as defined by sex, ethnicity, marital status, age, and geographic location. Multivariate analysis indicated that prevalence was higher among men, blacks, and unmarried men and peaked at ages 30-34. The crude incidence density was 0.20/1000 person-years, and multivariate analysis of incidence revealed patterns similar to those for prevalence. Comparison of prevalence and incidence data provides evidence that an increasing proportion of infection is occurring among women and in nonmetropolitan and small urban areas.