To observe the dynamics of pulmonary function in infants with respiratory syncytial virus (RSV) bronchiolitis. The to test the pulmonary function of 31 infants with RSV bronchiolitis during acute phase at admission was tested. Pulmonary fuction test was performed for the second time among 17 out of the 31 infants during the clinical recovery phase, and performed for the third time among 10 out of the 17 infants one week after discharge. In the meantime pulmonary function test was performed in 74 healthy infants as controls. During the acute phase of RSV bronchiolitis, the respiratory rate (RR), peak tidal expiratory flow (PTEF), peak tidal flow/tidal volume (PF/Ve), respiratory system resistance (Rrs), and functional residual capacity per kilogram (FRC/kg) were significantly increased, while the inspiratory tidal volume (Vi), inspiratory volume per kilogram (Vi/kg), expiratory tidal volume (Ve), percent of tidal volume to peak tidal expiratory flow (%V-PF), terminal flows/peak expiratory flow (25/PF), respiratory system compliance (Crs), and respiratory system compliance per kilogram (Crs/kg) were significantly decreased as compared with those in healthy infants. Ratio of mid-expiratory to mid-inspiratory flow (ME/MI) and respiratory system time constant (Trs) showed no statistically significant differences. The tidal breathing flow-volume (TBFV) loop displayed a concave expiratory curve. During the clinical recovery phase, the above abnormal indexes showed significant improvement, except for the %V-PF, 25/PF and Crs/kg which were still lower as compared with those in healthy infants, most indexes had returned to normal. The TBFV loop showed mild decelerating expiratory limb. One week after discharge all of the indexes returned to normal. In the acute phase of RSV bronchiolitis higher resistance in small airway and obstructive ventilatory disturbance can be seen. Some of the indexes remain abnormal during the clinical recovery phase. However, the abnormalities disappear in a short time. Pulmonary function test is a valuable way for surveillance of the course and prognosis of RSV bronchiolitis. %V-PF, 25/PF, and FRC/kg are sensitive indexes.