The serum concentration of YKL-40, a matrix protein of specific granules in neutrophils, was determined by RIA in 90 patients hospitalized with pneumonia of suspected bacterial origin. Of these, 64 were followed prospectively during antibiotic treatment with blood samples taken on day 0 (on admission and the start of treatment) and on days 1, 3, 5, 7, 10, and 21. Serum YKL-40 at admission was increased in patients with Streptococcus pneumoniae pneumonia (median, 893 microgram/L; 95% confidence interval [CI], 704-1560), compared with healthy subjects (median, 102 microgram/L; 95% CI, 64-247 microgram/L; P<.001) and in patients with pneumonia of unknown etiology (median, 448 microgram/L; 95% CI, 334-700; P<.05). Peak YKL-40 serum values were observed on day 1 and thereafter declined steeply to almost normal by day 3. During the first 10 days, there was a close relation between serum YKL-40 and markers of specific granules of neutrophils (serum lactoferrin and neutrophil gelatinase-associated lipocalin), which suggests that serum YKL-40 reflects exocytosis of specific granules of neutrophils in persons with acute bacterial pneumonia.