Male breast cancer is a rare disease and accounts for <1% of all breast cancers. Anterior lobe dysfunction of the pituitary gland is known to occur in only 15% of patients with pituitary gland metastasis, and diabetes insipidus occurs in 70% of patients in this subset. We report a case of pituitary metastasis in a patient with male breast cancer that resulted in anterior lobe dysfunction. A 64-year-old man presented with consciousness loss and gait disturbance. His serum sodium level was 117mEq/L. Magnetic resonance imaging revealed a suprasellar tumor that showed inhomogeneous enhancement and was attached to the optic chiasm. Diabetes insipidus was not evident at admission, but was observed immediately after the administration of the steroid hormone complement. The patient underwent subtotal resection of the tumor via a transsphenoidal approach. Pathological examination revealed metastasis from estrogen receptor-positive breast cancer. The patient underwent conventional post-operative radiotherapy combined with hormone replacement therapy and has remained free of symptoms for 16 months. Herein, we discuss the neuroendocrinology of and treatment for pituitary gland metastasis.