Objective:To investigate the effect of YKL-40 and tidal lung function in infants with bronchiolitis by measuring the serumYKL-40, tidal volume, the level of time to peak tidal expiratory flow (TPTEF), ratio of TPTEF to total expiratory time (TPTEF/TE), volume to peak expiratory flow (VPEF), and ratio of VPEF to total expiratory volume (VPEF/VE). Methods: A total of 176 patients admitted to Wuxi Children＇s Hospital from September 2013 to May 2014 were enrolled. Eighty-five hospitalized infants with bronchiolitis (40 cases in the mild group and 45 cases in the severe group), 44 common pneumonia, and 47 normal children were enrolled in this study. The levels of serum YKL-40 and the tidal volume, TPTEF/TE, and VPEF/VE were determined by tidal breathing flow-volume loops and analyzed by statistical analysis. Results: YKL-40 was higher in the group of bronchiolitis compared to that in the pneumonia group and the control group (P < 0.05). There were no statistically significant differences in tidal volume (F=0.90, P=0.408). There was a significant difference of the ratio of TPTEF to TE (TPTEE/TE) in the three groups (H=74.844, P < 0.0001). The TPTEE/TE in the severe bronchiolitis group was lower than that in the mild bronchiolitis group (χ2=8.78, P < 0.05), which at the same time had the lower level than the pneumonia group (χ2=27.50, P < 0.05). In the three groups, the ratio of VPEF to VE (VPEF/VE) showed statistically different (H=76.392, P < 0.001). The VPEE/EE in the mild bronchiolitis group was statistically higher than that in the severe group (χ2=6.80, P < 0.05), which was lower than that in the pneumonia group (χ2=32.70, P < 0.05). YKL-40 was low negatively correlated with TPTEF/TE and VPEF/VE, (r=-0.278,P=0.001;r=-0.272,P=0.002), respectively. Conclusion: YKL-40 maybe an important inflammatory marker in bronchiolitis. The tidal lung function was a good monitoring and judgment index for bronchiolitis.