Objective To investigate the serum level of surfactant protein D ( SP-D) in patients with chronic obstructive pulmonary disease ( COPD) and its clinical significance. Methods Serumlevels of SP-D in patients with acute exacerbations of COPD ( n = 29) , stable COPD ( n = 26) , and control subjects ( n = 19 ) were measured by ELISA. Multiple regression modeling was performed to determine the independent relationship between SP-D and lung function variables. Results The serum SP-D levels were significantly increased in the patients who experienced an acute exacerbation [ ( 70. 6 ±20. 7) ng/mL] compared with the patients with stable COPD and the control subjects [ ( 47. 9 ±13. 3) ng/mL and ( 31. 2 ±11. 4) ng/mL] ( both P lt; 0. 01) . The serum SP-D levels in the patients with stable COPD increased significantly than the control subjects ( P lt; 0. 01) . Smoking index and staging of COPD were positively related to SP-D level. Serum SP-D levels were also found to be inversely related to FEV1% pred in stable COPD. Conclusion Serum SP-D may be a potential diagnostic and staging biomarker for COPD.