Iatrogenic calcinosis cutis is a rare disorder that can be due to the intravenous administration of calcium or phosphate-containing infusions such as calcium gluconate or calcium chloride with extravasation. Fortunately, the course of calcinosis cutis is benign in immunocompetent children. The treatment remains supportive therapy. After about 6 months, there is no evidence of tissue calcification. We describe a 4-year-old girl with Acute Lymphoblastic Leukemia (ALL) who developed severe calcinosis cutis in the left humerus after extravasation of calcium gluconate during the treatment for the tumor-lysissyndrome-related hypocalcaemia. Surgical debridement and local wound care were not successful, and so a temporary suspension of chemotherapy was necessary to achieve complete healing of the lesion. Notwithstanding this complication, her ALL is in complete hematological remission after 8 months from the diagnosis. No functional or sensitive impairment due to the cutis has persisted.