A total of 205 infants who were hospitalized when younger than 3 months of age for pneumonitis were followed longitudinally. Of these patients, 145 (70%) had evidence of infection with one or more pathogens. The most common etiologic agents were Chlamydia trachomatis 61/193 (36%), respiratory syncytial virus 33/142 (23%), cytomegalovirus 42/203 (20%), Pneumocystis carinii 30/171 (17%), and Ureaplasma urealyticum 21/125 (16%). The initial clinical presentation was characterized by cough, rales, normal temperature, and diffuse obstructive airways disease by chest roentgenogram. Regardless of etiology, significant association occurred for cough and cytomegalovirus, apnea and Pneumocystis, and conjunctivitis and Chlamydia. Longitudinal follow-up demonstrates a mortality of 7/205 (3.4%). Morbidity was manifest as recurrent wheezing episodes in 86/187 (46%) patients, persistently abnormal chest roentgenographic findings for at least 12 months in 17/109 (15%) patients, and abnormal pulmonary functions in 15/25 (60%) patients. These abnormalities occurred irrespective of prematurity, atopy, or the initial etiologic agent associated with the pneumonitis. These data add further evidence that respiratory infections during infancy may well be predecessors of obstructive airways disease in later life.