Respiratory syncytial virus (RSV) is a common serious pathogen known to produce annual winter epidemics in young children. A 2-year study of children with significant respiratory disease during the summer revealed a 21% incidence of RSV infection. Respiratory secretions collected from ill children in the LSUMC outpatient clinics, from children seen by private physicians, and from children hospitalized with respiratory tract disease were assayed for RSV antigens. Approximately 39% of those surveyed in 1987 and 13% of those studied in 1988 were positive. As this prevalence was significant, we compared RSV-induced disease in 20 patients hospitalized in summer and 20 hospitalized in winter (1989). The patients were matched by age, weight, sex, and race. Comparisons included subjective severity of disease, presenting symptoms, physical findings, chest roentgenograms, treatment, and average length of hospital stay. No significant differences in disease severity and/or clinical presentation were found. Our findings show that RSV induces disease in the summertime more frequently than generally recognized, and severe disease requiring hospitalization is not infrequent. Physicians should consider RSV in children with serious respiratory disease throughout the year.