Because soldiers in the US Army are recurrently tested for the presence of antibody to the human immunodeficiency virus (HIV), HIV seroconversion rates can be directly measured. From November 1985 through October 1989, 429 HIV seroconversions were detected among 718,780 soldiers who contributed 1,088,447 person-years of follow-up time (HIV seroconversion rate, 0.39 per 1000 person-years). Period-specific seroconversion rates declined significantly from 0.49 per 1000 person-years (November 1985 through October 1987) to 0.33 per 1000 person-years (November 1987 through October 1988) to 0.29 per 1000 person-years (November 1988 through October 1989). The HIV seroconversion risk among active-duty soldiers was significantly associated with race/ethnic group, age, gender, and marital status. Based on these trends, we estimate that approximately 220 soldiers (95% confidence interval, 160 to 297 soldiers) were infected with HIV during 1989 and 1990, with potentially fewer in future years.