Abstract: Objective To investigate the effects of the age of mice when primarily infected with respiratory syncytial virus (RSV) on Th17/Treg balance and airway inflammation after second infection of RSV in adult mice. Methods Balb/c mice were divided into a neonatal infection group and a weaning infection group (n=36). After twice RSV nasal dripping, 6 mice were killed every other day at the eighth week. Then, lung tissue was taken, and the percentages of Th17 and Treg cells were detected by flow cytometry. ELISA was used to detect IL-17 concentration, while Western blot was used to detect the levels of RORγt and FOXP3 protein. Results The neonatal infection group presented remarkable increases in the percentage of Th17 cells, RORγt expression and IL-17 concentration in lung tissues, as well as marked decreases in the percentage of Treg cells and FOXP3 expression, compared to the weaning infection group (P<0.05). Substantially enhanced airway reactivity was found in the neonatal infection group, compared to the weaning infection group (P<0.05).The total number of cells, lymphocytes, neutrophils and eosinophils in BALF were significantly higher in the neonatal infection group than those in the control group (P<0.05). Conclusions RSV infection in neonatal mice may aggravate Th17/Treg balance and airway inflammation in secondary RSV infection in adult mice.