First hospitalizations (n = 1,293) for diabetes mellitus between 1974 and 1988 were used as a surrogate for insulin-dependent diabetes mellitus incidence among 17-34-year-old US Navy enlisted personnel followed for 6,077,856 person-years. In the 15-year period, the overall incidence of insulin-dependent diabetes mellitus was 21.3 per 100,000 person-years. Incidence did not differ significantly by sex, but was higher for blacks than whites (28.4 vs. 20.2 per 100,000 person-years, respectively; p < 0.05). Incidence increased with age threefold for white men and fivefold for black men (p < 0.05) between the ages of 17-19 and 30-34 years.