Background: Serum YKL-40 (chitinase-3-like-1) has recently been found to be either the cause or a biomarker for asthma. This study quantifies serum levels of YKL-40 in asthmatic children and evaluates the relationship between YKL-40 and asthma severity. Methods: We quantified serum YKL-40 levels in two groups of asthmatic children (aged between 2 and 12 years). Group 1 (n: 30) in between the attack (stable group) and group 2 (n: 30) during the exacerbation (acute attack group). Serum YKL-40 was measured by enzyme-linked immunosorbent assay (ELIZA) Kits (WKEA MED SUPPLIES CORP). We measured blood eosinophils and peak expiratory flow rate for children >5 years. Results: Serum levels of YKL-40 were significantly elevated in the acute asthma group and in the stable asthma group compared to the control group (673.92 ± 24.23 ng/ml, 552.73± 57.34 ng/ml, 136.17± 53.14) respectively, P < 0.001. YKL-40 levels were correlated positively with blood eosinophils in the stable group (q = 0.67, P