OBJECTIVES: M-ficolin (ficolin-1) is a complement-activating pattern-recognition molecule structurally related to mannan-binding lectin. It is produced by monocytes and neutrophils, and is found in serum. Its biological role is largely unknown. We assessed M-ficolin concentration in serum from pediatric cancer patients. The aim of this study was to explore association of M-ficolin with clinical and hematological parameters, and to investigate whether the risk of chemotherapy-related infections was related to M-ficolin concentrations in serum. METHODS: M-ficolin was measured by time-resolved immunofluorometric assay in serum taken at cancer diagnosis and was correlated with peripheral blood counts and bone marrow examinations performed at the same time. RESULTS: Median M-ficolin concentration in 94 children with cancer was 1.6μg/mL (interquartile range, 0.57-2.7; range, 0.055-25.8), and was not different from age-matched controls (median, 1.7μg/mL; p=0.92). M-ficolin was strongly associated with absolute counts of neutrophils (Spearman's rho, 0.45; 95%-CI, 0.26-0.65; p