Respiratory tract infections represented one of the commonest illnesses that occurred among U.S. Army personnel stationed in the Republic of Vietnam. Between 1965 and 1970 the years of this review, respiratory tract infections ranked approximately equal to diarrheal disease as a cause of hospitalization or assignment to quarters. Rates varied between 20 and 110 per 1000 troops per year. The specific casual agents responsible for acute respiratory diseases in Vietnam were not defined. Limited observations suggest that members of the adenovirus group and respiratory syncytial viruses were involved. During the fall of 1968, influenza due to the A2 Hong Kong strain (H3N2) was widespread, but it was not associated with marked increases in rates of hospitalization or mortality. Mycoplasma pneumoniae was the most common demonstrable causative agent in soldiers admitted to hospitals with pneumonia, 42% in one series.