Hypercalcemia is frequently observed in patients with breast carcinoma. When the biochemical effects of osteolytic metastases, hormonal therapy, or other pathological states are ruled out, simultaneous hyperparathyroidism must be considered. Six women with breast cancer were found to have anatomic parathyroid gland changes. Two of these patients showed hyperplasia. In one patient all of the parathyroid glands were involved, and in the other patient a single adenomatous gland was removed. The hyperparathyroidism of the latter patient disappeared postoperatively. In four of these patients a single adenoma was found. Their removal in three instances was associated with the disappearance of hyperparathyroidism. The fourth adenoma was found at necropsy.