Viral causes of acute lower respiratory tract infection were studied prospectively between 15 June and 31 October 1984 in 312 Filipino children less than 5 years old living in periurban slums and middle-class housing. The cause was based on viral antigen detection, virus isolation, and antibody assays. There were 131 children (41.2%) who were admitted to the hospital, and 150 (47%) had an infiltrate on chest radiograph. A total of 198 viral infections were confirmed in 162 patients (51.9%), 42.3% with single viral infection and 9.6% with mixed (two or more) infection. The infections were measles (21.4%), influenza A (15.9%), parainfluenza types 1, 2, and 3 (8.8%), respiratory syncytial virus (7.1%), influenza B (5.8%), enteroviruses (5.1%), adenoviruses (3.9%), herpes simplex virus (1.6%), and cytomegalovirus (1.3%). Viral infections other than measles were seen in 39.7% of the cases. The presence of viral infection correlated with better nutritional status. Influenza A or B diagnosis was associated with mild forms of acute respiratory tract infection, measles and a preceding rash with severe disease.