Objective: The studies on children with COVID-19 are very limited. The aim of this study is to reveal the effect of serum 25-hydroxy vitamin D level on clinical and laboratory parameters. Materials and Methods: The study included 74 children (35 boys and 39 girls) diagnosed with COVID-19. The retrospective data were obtained from the file records of the patients. Seventy-four patients were divided into 3 groups (group 1, deficient; group 2, insufficient; and group 3, sufficient) according to their serum 25-hydroxy vitamin D levels. Results: The mean age of all patients was 113.25 ± 64.55 months. The mean leucocyte count was substan- tially higher in group 3 compared to groups 1 and 2 (P = .05 and P = .002, respectively). The mean lympho- cyte and platelet count in group 3 was remarkably higher than both groups 1 and 2 (P = .001 and P = .002; and P = .04 and P = .01, respectively). The mean serum parathyroid hormone concentration in group 1 was markedly higher than both groups 2 and 3 (P = .003 and P = .002, respectively) while the mean serum 25-hydroxy vitamin D level in group 1 was remarkably lower than both groups 2 and 3 (P = .001 and P = .001, respectively). Serum 25-hydroxy vitamin D concentrations were positively correlated with leucocyte, lymphocyte, and platelet counts (r = 0.221, P = .05; r = 0.396, P = .001; and r = 0.249, P = .03, respectively) while there was a negative correlation with parathyroid hormone concentrations (r = −0,436, P = .001). Conclusion: This study suggests that COVID-19 has a benign course in children and that serum 25-hydroxy vitamin D concentration may have a role in the lymphocyte count.