Although enhanced immune reaction caused by the respiratory syncytial virus (RSV) in allergen-sensitized animal model has been reported, RSV illnesses in children already sensitized or having recurrent wheezing episodes have not been completely studied. In addition, the reason for male dominances in RSV infection at young ages was also inconclusive. Therefore, gender analysis in recurrent wheezing children with RSV infection can shed light on asthma pathogenesis. We studied the clinical features and the laboratory data of RSV infections in children who had recurrent wheezing histories. The subjects with RSV infection consisted of 98 boys and 58 girls. The children under 4 yr of age were 123 (78.8%) in number. Children with pneumonia were 78 and those with febrile episode were 119. Children above 1 yr of age were highly sensitized with mite antigen (75/96, 78.1%). The clinical symptoms and signs differed according to their ages. Children in each age group behaved differently in their immune reaction to RSV. Above all, 3-yr-old children deteriorated clinically during acute RSV infection, accompanied by transient elevated C-reactive protein (CRP) and suppressed blood eosinophil counts. Clinical features differed in several points between boys and girls. In general, the white blood cell count and the CRP levels were higher in girls in every age group. Blood eosinophil counts at the acute illness were significantly higher in boys than girls aged 2 and 3< yr. Age and gender comparison in already sensitized children might suggest a clue to asthma pathogenesis.