Although hypercalcemia is a well-known complication of malignant diseases, hypocalcemia seems to be a rather rare one. A 34-yr-old woman with advanced breast cancer who presented hypocalcemia is described. She had generalized multiple osteolytic bone metastases which were progressive in spite of chemo-endocrine and radiation therapy. She was admitted because of severe bone pain and dyspnea caused by bilateral pleural effusion. Laboratory examination on admission showed that the serum calcium was 9.6 mg/dl, serum total protein 5.9 g/dl, serum inorganic phosphorus 4.6 mg/dl, and serum alkaline phosphatase 29.6 King-Armstrong units. The serum calcium gradually fell to 7.0 mg/dl on the 45th hospital day when the serum total protein was 6.8 g/dl and she complained of paresthesia in the extremities. On the 58th day, severe tachycardia and hypotension developed and she died of congestive heart failure on the 67th hospital day. At that time the serum calcium was 5.4 mg/dl. During her hospital course, the plasma parathyroid hormone levels were examined repeatedly and were 0.4, 0.6, 0.6 and 0.7 ng/ml (normal; less than 0.5 ng/ml). Autopsy revealed that cancer invaded the space between the thyroid and the trachea and no parathyroid glands could be found even in the mediastinum. Microscopically the parathyroid glands were replaced completely by the cancer cells. These observations indicate that metastasis of breast cancer to the parathyroid glands caused relative hypoparathyroidism, resulting in hypocalcemia. In addition, congestive heart failure which was refractory to digitalis and diuretics might have been caused by impaired contractility of the myocardium associated with hypocalcemia.