The clinical and pathological findings in 2 cases of delayed radionecrosis of the brain following the irradiation of the contents of the pituitary fossa are described. They consisted of the recurrence of visual symptoms and the recurrence or onset of hypothalamic symptoms; of visual field defects unlike those usually seen in the recurrence of a pituitary adenoma; of necrosis of the hypothalamus, the inferomedical portions of the frontal lobes, the uncal gyri and the optic chiasm. The necrotic areas were characterized by the presence of a widespread, dense exudate of fibrin and degenerative and occlusive changes in the vessels. A region of degenerative change of grey and white matter containing narrowed and occluded vessels, lay adjacent to the central necrotic regions. The possible pathogenesis of delayed radiation necrosis on an allergic basis is discussed. The reasons why therapeutic irradiation of the pituitary region is liable to cause delayed necrosis of the hypothalamus are discussed. The difficulty in the differential diagnosis of the symptoms of delayed radionecrosis and recurrence of a pituitary tumour is mentioned. The changes in the visual fields were unlike those usually seen in tumour recurrence and were similar to those sometimes seen in vascular disease of the optic nerves and chiasm.