Hypocalcemiccrisis developed in a patient with monophasicsynovial sarcoma after amputation of the right leg, followed by long-term treatment with cisplatin and doxorubicin. Laboratory data revealed severe hypomagnesemia and hypocalcemia. High normal intact parathyroid hormone (PTH), elevated mid-region PTHand undetectable osteocalcin levels had already been found before the appearance of obvious symptomsconcomitantly associated with moderate hypomagnesemia and hypocalcemia. Further, both PTHlevels measured by two different methods gradually decreased until the initiation of magnesium supplementation. The magnesiumsupplement immediately relieved the tetany, and induced striking increases in both intact and mid-region PTHlevels transiently and continuous elevations of osteocalcin levels. These results suggest that magnesiumdepletion has dual effects on PTHsecretion, from stimulation to inhibition, as hypomagnesemiaprogresses. Both relative hypoparathyroidism and refractoriness of bone to PTH were thought to be responsible for hypocalcemia due to hypomagnesemia.