Aim: Mannose-binding lectin (MBL) is an important component of the natural immune system. Its low levels have been linked to increased frequency of opportunistic infections. This study aimed to determine the association of serum MBL levels and duration of febrile neutropenia (FN) after cytotoxic chemotherapy in patients with acute leukemia. Material and Methods: Seventy patients aged 15-75 years with acute leukemia (40 AML (Acute myeloid leukemia), 30 ALL (Acute lymphoblastic leukemia)), and 30 age-matched healthy subjects were included in this study. Blood MBL levels were measured using ELISA Kit before chemotherapy (MBL1) and at FN (MBL2). Results: The MBL1 levels of the patient group (Median 466 ng/ml; interquartile range (iqr): 4507) were higher than that of the control group (Median 485 ng/ml; iqr: 1872), but this difference was statistically insignificant (p=0.92). During FN, MBL levels of 49 (70%) patients increased and 21 (30%) patients remained at the same level. The MBL2 levels (Median 772 ng/ml; iqr: 5870) of the patients were significantly higher than the MBL1 levels (p< 0.001). Patients with very low (<100 ng/ml) MBL2 levels had significantly longer FN duration than patients with normal (>1000 ng/ml) MBL2 levels (p=0.016). Discussion: Our results suggest that duration of FN is longer when MBL level is low especially less than 100 ng/ml. These patients seem to have the highest risk for infection-related morbidity and mortality and deserve interest for trials of MBL replacement.