A 66-yr-old man presented with presyncopal episodes, dizziness, anorexia, nausea, and weight loss and was noted to have low blood pressure with a postural drop and sparse eyebrows. Laboratory investigations revealed evidence of hypopituitarism. Magnetic resonance imaging (MRI) revealed a non-enhancing mass arising from the adenohypophysis. The neurohypophysis was displaced laterally but appeared otherwise normal. The lesion was thought to be a nonfunctioning pituitary adenoma and a trans-sphenoidal hypophysectomy was performed. Histologically, this lesion was a diffuse large B-cell lymphoma that had features of a high grade mucosa associated lymphoid tissue (MALT)-type lymphoma arising in association with low-grade B-cell lymphoma of MALT type. There was no clinical or radiological evidence of lymphoma in other sites and there was no evidence of an immunocompromised state. Only one previous case of primary malignant lymphoma of the pituitary has been reported and this patient presented with compression of the optic chiasm. We describe the clinical and pathological features of a patient who presented with hypopituitarism and was found to have a pituitary lymphoma. This is the first reported case of a pituitary lymphoma presenting with pituitary failure and the first case characterized by lymphocyte-marker studies that confirmed it to be a B-cell lymphoma.