From January 1, 1967 to December 1971, the temporal pattern of respiratory syncytial virus infection was investigated in infants and children younger than 18 months hospitalized for acute lower respiratory tract disease. Of 4696 infants and children with acute lower respiratory tract disease admitted to the Cook County Hospital, 2530 were tested for virus infection by virus isolation or serologic procedures or both. Overall, respiratory syncytial virus infections were detected in 12% and parainfluenza 3 virus in 10.8% of individuals tested. Other respiratory viruses were less commonly identified. Respiratory syncytial virus epidemics occurred annually and were temporally synchronous with the peak periods of respiratory disease admissions. Only during epidemics of respiratory syncytial virus did admission for respiratory tract disease usually reach 40 patients or more weekly. The peak months of respiratory syncytial virus epidemics were December 1966, January 1968, February-March 1969, April 1970 and January 1971. Epidemics lasted about 17 weeks. No similar annual epidemic pattern was seen with the other myxoviruses.