From 1986 through 1988, the US Navy and US Marine Corps administered 1,956,631 enzyme-linked immunosorbent assay screening tests for antibodies to the human immunodeficiency virus to 1,070,511 active-duty enlisted and officer personnel. This study identified all persons who had an initial test with negative results. This population was then followed up and those who later tested positive for human immunodeficiency virus were identified as seroconverters. There were 582 seroconversions identified from a total of 987,479 person-years at risk. The age adjusted seroconversion rate and 95% Poisson confidence intervals for navy personnel was 0.69 per 1000 person-years (95% confidence interval, 0.63 to 0.76). Age-adjusted rates in men were 5.0 times those of women. Age-adjusted rates in blacks were 3.7 times those of whites. The age-adjusted seroconversion rate in Marine Corps personnel was 0.28 per 1000 person-years (95% confidence interval, 0.22 to 0.36). Similar demographic patterns were present in the Marine Corps and the US Navy. This study is one of the first reports of incidence of human immunodeficiency virus seroconversion by demographic characteristics in a large, young, and apparently healthy population.