YKL40 is secreted by activated macrophages and neutrophils. Elevated serum concentrations of YKL40 are found in patients with diseases characterized by inflammation or ongoing fibrosis. The aim of this study was to evaluate serum YKL-40 levels in patients with Streptococcus pneumoniae bacteremia and to correlate these levels with clinical findings and outcomes. YKL40 was determined by ELISA and 89 patients were included in the study. Serum YKL-40 levels were significantly higher in patients with S. pneumoniae bacteremia (median 342 microg/l; range 20-20,400 microg/l) than in age-matched healthy subjects (44 microg/l; 20-184; p < 0.001). Serum YKL-40 levels were related to the severity of the infection, with significantly higher serum YKL-40 levels being observed in patients who needed hemodialysis (p < 0.001), pharmacological treatment of hypotension (p < 0.001) and mechanical ventilation (p = 0.003) compared to those in patients who did not need this supportive treatment. Nineteen patients died and these patients had significantly higher serum YKL-40 levels (980 microg/l; 88-20,400 microg/l) than those of survivors (256 microg/l; 20-9,100 microg/l; p < 0.001). Serum YKL40 level was an independent prognostic factor of survival in logistic multivariate regression analysis (p = 0.002). In conclusion, high serum levels of YKL40 indicated a poorer prognosis for patients with S. pneumoniae bacteremia.