We present an unusual case of primary central nervous system (CNS) lymphoma presenting with bilateral symmetric hypothalamic lesions causing diabetes insipidus and hypopituitarism. A 50-year-old male presented initially with mental status changes, polyuria and polydipsia. The patient was determined to have diabetes insipidus (DI) and significant anterior pituitary deficiencies resulting in symptomatic pleural and pericardial effusions. Brain MRI with contrast demonstrated bilateral enhancement of his hypothalamus extending to the optic tract. The extensive diagnostic workup that ensued on his initial presentation was non-diagnostic as he had no obvious site of involvement that was easily accessible to biopsy. With close follow-up, the patient had rapid radiographic progression of his disease to his cerebral hemispheres. He therefore underwent brain biopsy and was diagnosed with primary CNS large B cell lymphoma. Chemotherapy has resulted in disease remission with resolution of MRI findings, but the patient has not had resolution of the hypopituitarism or DI. This case highlights the unique diagnostic challenge of patients with isolated hypothalamic lesions.