Objective: The aim of this study was to investigate vitamin D concentration among children hospitalized in University Hospital Center Split with newly diagnosed acute lymphoblastic leukaemia (ALL) and compare it with recommended vitamin D concentrations for healthy pediatric population. Patients and methods: Patients were children hospitalized at the Department of Pediatrics at University Hospital Center Split in the period between 1st September 2013 and 30th June 2017 with diagnosis of ALL. Research was conducted by retrospective cross-sectional analysis of medical records of Department of Pediatrics at University Hospital Centre Split. Serum vitamin D concentration was measured using a commercially available Elecsys® Vitamin D total assay. Ethics Committee of the University Hospital Centre Split approved the study. Results: Vitamin D sufficiency among children suffering from ALL was found in 2 (10%) patients. Low vitamin D levels were present in 90% patients; 20% patients had vitamin D insufficiency and 70% had vitamin D defficiency. Mean vitamin D concentration in children suffering from ALL in this study was 42.6 nmol/L. Considering sex, mean vitamin D concentration among male patients was 47.5 nmol/L and 36.6 nmol/L among female patients. Mean vitamin D concentration in group of patients aged between 1 and 9 years old was 40.6 nmol/L, in group of patients aged between 10 and 15 years old that number was 47.7 nmol/L and mean vitamin D concentration was 43.0 nmol/L in children older than 15 years old. Conclusion: We have come to a conclusion that 90% children with newly diagnosed ALL have low vitamin D concentration. Vitamin D deficiency of our patients was not developed due to therapy because vitamin D concetrations were measured before the treatment started. It is recommended to control vitamin D level in children with ALL during therapy and if necessary, supplement it.