Since genetic polymorphisms vary among populations, and the possible risk factors for GC-induced-ON has not been studied in the Egyptian population, therefore, we performed this study to assess the frequency of pharmacogenetic variants of VDR fok1 and TYMS genes among the studied Egyptian pediatric ALL patients and to explore their association with the occurrence of osteonecrosis. The gene discussed is VDR; the disease is acute lymphoblastic leukemia.