Background: The vitamin D deficiency is ubiquitous among children population. Similar problem is recognized among paediatric patients with acute lymphoblastic leukaemia. The purpose of this study was to analyze the prevalence of vitamin D deficiency and investigate the connection between vitamin D status and course of induction treatment of ALL. Methods: A cross-sectional study including 59 patients with newly diagnosed ALL from May 2017 until November 2020. Results: The vitamin D insufficiency was found in 36% of patients. There were no seasonal differences in vitamin D status. Patients with optimal 25(OH)D concentration presented more profound thrombocytopenia (p=.015) and required more frequent platelet transfusions (p=0.018). The good prognosis factors as B phenotype and hyperdiploidy were also more frequent among children with higher 25(OH)D concentration (p=0.01 and p=0.014, respectively). Conclusion: The study displayed that patients with higher serum concentration of 25(OH)D presented deeper thrombocytopenia and needed more frequent transfusions. Moreover, those patients showed higher rates of B-cell leukaemia and hyperdiploid karyotype. We did not find any influence of potential sunlight exposure (defined as the season of the year at admission) on serum 25(OH)D, which supports the supplementing of vitamin D throughout the entire year. Moreover, the supplementation of vitamin D seems to be safe in the context of renal complications connected to calcium and phosphorus imbalance as no correlation between their levels and 25(OH)D concentration was found.