YKL-40 (chitinase-3-like-1) has been introduced as a marker of inflammation in asthma. The aim of this study was to determine the role of YKL-40 in asthma and to evaluate the relationship between YKL-40 and asthma severity.In the study, 60 non-smoker asthma patients without additional diseases (aged between 20-60 years, female: 34) were grouped [Group I: Well controlled asthma patients (n: 30), Group II: Patients during acute exacerbation of asthma (n: 30)]. Healthy non-smoker female individuals were included in Group III (n: 30) as a control group. The level of serum YKL-40 of all groups were determined by ELISA. Also, serum YKL- 40 level was correlated with age, asthma duration in years, body mass index (BMI), forced expiratory volume in first second/ forced vital capacity (FEV1/FVC, %), FEV1 (%), and total IgE levels of asthma patients. Mean serum YKL-40 level was highest in patients during acute exacerbation of asthma (36.36±10.49 ng/ml) while mean serum YKL-40 level was the lowest (13.20±5.60 ng/ml) in the control group. There was a negative significant correlation between the serum YKL-40 levels and FEV1 (%) in patients during acute exacerbation of asthma. There were no significant correlations between the serum YKL-40 levels and other variables in group II.We found that increased serum YKL-40 levels may be used as a marker for evaluation of asthma severity and genetic polimorphism.