Age-adjusted rates of total first hospital admissions of U.S. Navy enlisted personnel with diagnoses of infectious and parasitic diseases declined from a high of 112.9 per 10,000 person-years in 1977 to a low of 50.3 per 10,000 person-years in 1982. Ten specific diagnoses accounted for 78% of these hospitalizations. Eight diagnoses exhibited significant increases in rates over the study period. Improved medical care and prophylaxis, greater emphasis on outpatient care, changing social characteristics of Navy personnel, and changes in deployment theaters may account for the observed trends in hospitalization.