Background: There is a paucity of lung specific biomarkers to diagnose exacerbations of chronic obstructive pulmonary disease (COPD) and to track their progression. Surfactant protein D (SP-D) is a pulmonary collectin regulating the innate immunity of the lung and its serum expression is perturbed in COPD. However, it is not known whether serum levels change during exacerbations. We sought to determine whether serum SP-D levels are raised in COPD exacerbations.Objectives: To determine whether or not patients with exacerbations have elevated serum SP-D levels compared with asymptomatic controls, stable disease.Study design: case control study.Methods: We measured serum SP-D levels from patients with stable COPD (n = 14), patients experiencing acute exacerbations (n = 13) and in control subjects (n = 54) using a specific immunoassay and compared the levels using analysis of variance.Results: Serum SP-D levels were significantly increased in patients who experienced an acute exacerbation (227 ± 120 ng/mL) compared to patients with stable disease (151 ± 83 ng/mL) or control subjects (128 ± 65 ng/mL; p = 0.003). Serum SP-D levels were also found to be inversely related to various lung function parameters including FEV1/FVC% predicted.Conclusions: Our study suggests that serum SP-D levels are increased in patients during exacerbations and may be a potential diagnostic biomarker for COPD exacerbations.