A 9-year-old girl presented at the Pediatric Emergency Department with an acute onset of gastrointestinal symptoms due to hypercalcemia. Despite the absence of circulating blast, bone marrow biopsy was diagnostic of acute lymphoblastic leukemia. The hypercalcemia was initially treated with intravenous hydration and furosemide, and later on with bisphosphonates. However, the serum calcium levels normalized only after the beginning of specific chemotherapy. Hypercalcemia represents an emergency in children, and acute leukemia must be considered in its differential diagnosis, even when there are no circulating blasts.