To study the incidence of nosocomial respiratory tract infection along the first week after admission, 31 children under 2 years of age admitted to the pediatric wards of a general hospital at Metropolitan Santiago, Chile, with the diagnosis of lower respiratory tract infection, were studied for viral agents by immunofluorescence tests, viral isolation from pharyngeal aspirates, paired serum viral antibodies and bacterial cultures, all of them performed at admission and 4 to 5 days later from May through August 1988. In 13 out of 31 patients admitted because of acute lower respiratory tract infection at least one new virus (to a total of 18 viral isolates) was detected in the second sample, which could have been nosocomially acquired, as follows: adenovirus from 8 cases, respiratory syncytial virus from 5 patients and cytomegalovirus from 5 patients, in different combinations. No significant changes in bacterial cultures were found.