Breast cancer is the malignant neoplasm most commonly associated with hypercalcemia. In breast cancer the majority of the hypercalcemia cases result from osteolytic metastatic bone disease of the primary tumor. In a few patients hypercalcemia results from other conditions like primary hyperparathyroidism. Here, we present two female patients who were treated for breast cancer. Hypercalcemia in these two patients was diagnosed as being due to primary hyperparathyroidism. One of them was submitted to surgery and the calcium level dropped to the normal level thereafter. The other one refused surgery and was treated with biphosphonate and calcitonin. We suggest that when hypercalcemia occurs in breast cancer, primary hyperparathyroidism should be considered as possible cause.