Acute respiratory infections have been found to be a common cause of morbidity in young adults in the tropical setting of the Philippines, with incidence rates equalling those of colder northern countries. Influenza outbreaks have occurred as part of a worldwide pattern and parainfluenza infections are common, as revealed by serological tests on such students. About half of the respiratory infections could be identified etiologically. In contrast, clinical infections from group A haemolytic streptococci, Mycoplasma pneumoniae, adenoviruses, and infections due to Epstein-Barr virus (infectious mononucleosis) were rare in this population. The role of prior immunity is discussed in relation to these patterns.