Surfactant protein (SP)-D is a lung-derived protein that has been proposed as a biomarker for inflammatory lung disease. Serum SP-D was evaluated as a biomarker for components of chronic obstructive pulmonary disease (COPD) in the Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints (ECLIPSE) cohort and its response assessed to the administration of the anti-inflammatory agent prednisolone. The median level of serum SP-D was significantly elevated in 1,888 individuals with COPD compared to 296 current and former smokers without airflow obstruction (121.1 and 114.3 ng x mL(-1), respectively; p = 0.021) and 201 nonsmokers (82.2 ng x ml(-1); p<0.001). There was no correlation with the severity of COPD. Individuals with COPD who had a serum SP-D concentration that was greater than the 95th percentile of nonsmokers (175.4 ng x mL(-1)) showed an increased risk of exacerbations over the following 12 months (adjusted OR 1.30; 95% CI 1.03-1.63). Treatment with 20 mg x day(-1) prednisolone for 4 weeks resulted in a fall in serum SP-D levels (126.0 to 82.1 ng x mL(-1); p<0.001) but no significant change in post-bronchodilator forced expiratory volume in 1 s. Serum SP-D concentration is raised in smokers and may be useful in identifying individuals who are at increased risk of exacerbations of COPD. It may represent an intermediate measure for the development of novel anti-inflammatory agents.