The authors identified hospitalizations for pneumonia (n = 6,522) in active-duty Navy and Marine Corps personnel during 1981-1991 from computerized inpatient records. The crude mean annual rate of pneumonia hospitalization was 77.6 per 100,000 active-duty personnel; 65% of pneumonia hospitalizations had no etiologic agent identified. The most commonly reported agents to cause pneumonia hospitalization were Streptococcus pneumoniae (12.3%), Mycoplasma pneumoniae (10.8%), other streptococcal species (2.1%), and Haemophilus influenzae (1.9%). The median age at hospitalization was 22 years. The median duration of hospital stay was 4 days and the case fatality rate was 0.4%. The authors used a 2% sample of the entire population and by means of stepwise unconditional multivariate logistic regression modeling for pneumonia found that, independent of age, the most junior Navy and Marine Corps personnel were at highest risk. Whites were at higher risk than blacks, Hispanics, or Filipinos. These results indicate that among this generally healthy US young adult military population, pneumonia hospitalization is common, often brief, and frequently without specifically identified pathogens.