We report a 4-month-old male presenting with hypocalcemia, hyperleukocytosis, and newly diagnosed acute myeloid leukemia. He received chemical leukoreduction through low-dosed cytarabine, with appropriate decrease in his white blood cell count and development of subsequent symptomatic hypocalcemia, which did not normalize until the underlying vitamin D deficiency was addressed. A single dose of cholecalciferol raised the serum calcium concentration to an appropriate level within 48 hours. For infants with newly diagnosed leukemia and prolonged hypocalcemia, vitamin D deficiency should be considered in the differential diagnosis and managed appropriately.