Humoral hypercalcemia of malignancy (HHM) is a rare complication of malignant pediatric tumors, specifically those that secrete humoral factor(s), such as parathyroid hormone-related peptide (PTHrP). The authors report a case of severe hypercalcemia associated with ovarian dysgerminoma in a 10-year-old girl. In this case, the humoral factor was considered to be 1,25-dihydroxyvitamin D. HHM is extremely resistant to medical therapy. Therefore, tumor resection or volume reduction is necessary to control serum calcium levels.