Rarely, primary carcinomas can metastasize to the pituitary. Mostly the posterior lobe is affected and diabetes insipidus can develop. We report a 47-year-old woman who was on follow-up for metastatic breast carcinoma and admitted to the endocrinology unit for polyuria and polydypsia, which had worsened gradually. Her laboratory findings were compatible with diabetes insipidus. Magnetic resonance imaging revealed a huge mass involving the suprasellar cistern, surrounding the cavernous sinus and compressing the optic chiasm and infundibulum. Radiotherapy for the sellar region and chemotherapy were performed and desmopressin was started. The symptoms were controlled with desmopressin. Increased amounts of urine output and oral fluid intake in a patient with a diagnosis of carcinoma should alert the physicians of a possibility of pituitary metastasis, and the hormonal insufficiency should be corrected to increase the patient's quality of life.