Context: Incidence rates of asthma in young adults appear to have been rising in the general population. Time trends and demographic risk factors are reported here for asthma in a well defined population at risk of U.S. Navy young to middle-aged adults.Objective: To measure time trends and demographic risk factors for asthma.Design: Incidence rate survey of first hospitalizations for asthma in a defined m.ilitary population at risk.Setting: U.S. Navy and its worldwide medical care system. Includes 4,809,422 person-years at risk among shore-based Navy enlisted service members during 1980-1996.Patients: Navy enlisted personnel first hospitalized for asthma (ICD9 Code 493) during 1980-1996.Main outcome measure: Incidence rate of first hospitalization for asthma.Results: There were 2,320 first hospitalizations for asthma during 1980-1996, including 1,648 men and 672 women. The incidence rate of first hospitalizations for asthma was 35.4 per 100,000 person-years (95% confidence interval 33.2-37.6) in men and 109.7 per 100,000 person-years (95% CI, 97.3-138.8) in women during 1980-1989. It was 39.2 per 100,000 (95% CI, 36.7-41.8) in whites and 58.1 per 100,000 (95% CI 50.9-66.1) in blacks. Annual incidence rates in men rose from 42.2 per 100,000 (95% CI, 34.6-51.6) in 1980 to 49.7 per 100,000 (95% CI, 40.7-60.6) in 1996, while rates in women rose from 151.3 per 100,000 (95% CI, 99.7-220.9) in 1980 to 265.9 per 100,000 (95% CI, 210.1-335.0) in 1996.Conclusions: There was a steep rise in incidence rates of hospitalized asthma in women during 1980-1996, and a moderate rise in rates in men. Incidence rates in wornen were approximately five times those in men in recent years. u