Hypersecretion of PTHrP is a relatively common cause of malignancy-related hypercalcemia. However, there is only one case report of letrozole induced hypercalcemia. A 52-year-old female patient was referred to our clinic because of the recent discovery of hypercalcemia (11.0 mg/dL). The patient had a history of left breast carcinoma. She had started a course of letrozole (aromatase inhibitor; 2.5 mg dose/day) ten months earlier. Patient's parathyroid hormone-related protein levels were normal and a bone scintigram revealed no evidence of skeletal metastasis. Other potential causes of high calcium levels were ruled out. We recognized that, when letrozole was taken at one dose daily (2.5 mg), she had recurrent hypercalcemia. Our experience suggests that letrozole may precipitate hypercalcemia in a patient with breast cancer.