&NA;An 18‐year‐old woman presented with visual disturbance and endocrine dysfunction (diabetes insipidus, delayed puberty, hypothyroidism, hypoadrenalism, and hyperprolactinemia). Computed tomography and enhanced cisternography showed a single hypothalamic mass, which proved at biopsy to be histiocytosis X. Further studies showed the disease to be restricted to the hypothalamus. The patient was treated with hormonal replacement therapy, but her visual acuity continued to worsen. Visual acuity improved after low‐dose irradiation of the pituitary fossa and hypothalamus, which also led to disappearance of the mass. This case shows that localized hypothalamic histiocytosis X can be diagnosed and treated successfully before other systems become involved. (Neurosurgery24:93‐95, 1989)