Hypercalcemia is rarely observed in acute lymphoblastic leukemia. So far, all cases presenting with hypercalcemia are pre-B cell ALL. In this case, a mature B-cell ALL patient presenting with hypercalcemia is discussed. A threeyear-old boy had a history of fever, weakness, swelling and pain in both kness. In the patient?s blood smear, 93% L3 type blasts were seen and 90% L3 type large blasts with vacuoles were seen in the bone marrow aspiration smear. The flow cytometry results were as follows: CD10: 87%; CD19: 85%; KAPPA: 66%; and Lambda: 35% was compliant with mature B cell ALL. His calcium level: 15 mg/dl. Although previous cases imply that hypercalcemia is usually treated with pamidronate, calcium levels gradually decreased to normal levels within five days with intravenous fluid therapy, furosemide, and steroids in our case. Conclusion: Hypercalcemia is rare in leukemia. So far, all cases presenting with hypercalcemia are pre-B cell ALL. It is important to know that hypercalcemia could be seen in the mature B-cell ALL.