Respiratory syncytial virus is the most important cause of serious lower respiratory tract infection in children. For children followed up from birth in the Houston Family Study, the infection rate was 68.8/100 children less than 12 months of age and 82.6/100 during the second year of life. Virtually all children had been infected at least once by 24 months of age, and about one half had experienced two infections. Although lower respiratory tract disease (LRD) was common (22.4/100 during year 1 and 13.0/100 during year 2), most children had only one LRD illness. The risk of reinfection was inversely related to the level of neutralizing antibodies in the serum. Reinfection illnesses were generally mild, and risk of reinfection decreased to only 33.3/100 during year 4. Studies of children with LRD and surveys of hospitalizations provide the basis for an estimate of the number of children hospitalized each year during the respiratory syncytial virus epidemics. Almost 100,000 children in the United States experience an illness of sufficient severity to require hospitalization.