A 68-year-old man with undifferentiated carcinoma occurring in the cardioesophageal junction accompanied by hypercalcemia is reported. The serum level of parathyroid hormone-related protein (PTHrP) was remarkably elevated. Serum calcium and PTHrP levels decreased following chemotherapy, but this amelioration was temporary. He died of hypercalcemia. On autopsy, a significant amount of immunoreactive PTHrP was detected in the tumor tissue extract, and the tumor cells were stained strongly positive for PTHrP by immunohistochemistry. This is the first case of undifferentiated carcinoma in the gastrointestinal tract which demonstrated hypercalcemia due to PTHrP produced by the malignant tumor.