Background and objective: Some published data suggest that 25(OH) vitamin D (vitamin D) may participate in the airway inflammation of obstructive airway diseases. This study was performed to evaluate the relationship between serum vitamin D levels and lung functions in patients with obstructive airway disease including COPD and bronchial asthma in real clinical practice. Methods: COPD was defined by pulmonary function, FEV1/FVC of ≤ 70 % and FEV1 of ≤ 80% of predicted value. Bronchial asthma was diagnosed by positive airway hyperresponsiveness or bronchodilator responsiveness. Serum vitamin D was measured by chemoluminescence immunoassay. All values are expressed as mean± SD. Results; 29 patients (48.3%) were compatible with COPD, vitamin D level were 25.83±13.06 ng/ml and 31 patients (51.7%) were compatible with bronchial asthma, their vitamin D level were 35.47±31.19 ng/ml. However, there was no significant difference in serum vitamin D levels between two groups (p>0.05). Vitamin D insufficiency (≤ 20 mg/ml) was observed in 10 patients (34.48%) of COPD and 10 patients (32.35%) of asthma (p>0.05). COPD with reversible component in pulmonary function was demonstrated in 6 patients (10.9%), their mean vitamin D level was 29.73±13.57 ng/ml. There was no significant difference in the serum level of vitamin D between copd with or without reversible airway obstruction (p>0.05) Serum vitamin D level and lung function showed some trend of positive correlation, however, we could not find statistical significance (r=0.18, p>0.05) Conclusions: Reduced vitamin D levels seem to be associated with impaired lung function. Further studies are required to evaluate the relationship between reduced vitamin D level and impaired lung function in patients with obstructive airway diseases.