Mannan-binding lectin (MBL) is a key components of the lectin-pathway of complement-activation. only little is known on the influence of the it's role in cancer suppression and promotion . The lectin pathway of complement activation is an important part of innate immunity. The complement system provides immediate defense against infection and has proinflammatory effects. Our study aimed to assess the relationship between MBL serum levels before and after chemotherapy in Syrian children with acute lymphoblastic leukemia, aiming to find a relation, which serve in early forecast of patients response to chemotherapy, where the study has been done at Children University Hospital in Damascus between 2013-2014. This study included 37 children diagnosed with acute lymphoblastic leukemia (ALL). The levels of Mannose-Binding Lectin (MBL) were determined by Enzyme Linked Immuno Sorbent Assay (ELISA). The result was that serum levels of MBL was strongly reduced after chemotherapy in children with acute lymphoblastic leukemia, and a strong correlation between serum marker levels before and after chemotherapy was found.