Deficiency of one or more anterior pituitary hormones may follow treatment with external irradiation when the hypothalamic-pituitary axis falls within the fields of irradiation. Hypopituitarism occurs in patients who receive radiation therapy for pituitary tumours, nasopharyngeal cancer and primary brain tumours, as well as in children who undergo prophylactic cranial irradiation for acute lymphoblastic leukaemia, or total body irradiation for a variety of tumours and other diseases. The degree of pituitary hormonal deficit is related to the radiation dose received by the hypothalamic-pituitary axis. Thus, after lower radiation doses isolated growth hormone deficiency ensues, whilst higher doses may produce hypopituitarism. The timing of onset of the radiation-induced pituitary hormone deficit is also dose-dependent. The main site of radiation damage is the hypothalamus rather than the pituitary, although the latter may be affected directly.