SP-D is a lectin involved in the first line of defense against microorganisms. It is primarily found in the lung but also at extrapulmonary sites and in the circulation. An immunoassay for the quantification of SP-D in serum was established and the SP-D concentration was measured in consecutive blood samples from 61 patients hospitalized for community-acquired pneumonia of suspected bacterial origin. On the day of admission to the hospital the serum SP-D concentration was significantly lower than that in healthy subjects. On day 5, the SP-D concentration had increased on average three times the concentration on admission and then slowly declined toward normal levels. CRP was measured simultaneously but no correlation was observed between concentrations of SP-D and CRP. The results show a wide range of serum SP-D concentration in healthy volunteers and indicate that significant changes occur during pulmonary infection.