Endocrine complications after radiotherapy for tumors of the head and neck are thought to be relatively rare. The availability of synthetic hypothalamic hormones for clinical investigations and the radioimmunoassay of hormones have enabled us to study function of the hypothalamic pituitary axis in 15 patients who had radiotherapy for nasopharyngeal cancer. Fourteen had evidence of endocrine deficiency. Twelve patients had evidence of hypothalamic dysfunction, 7 developed primary pituitary hormone deficiencies, and 3 developed primary hypothyroidism. These results indicate that [1] secondary hypopituitarism due to a hypothalamic lesion after radiotherapy for nasopharyngeal cancer may be more common than suspected in the past; [2] primary hypopituitarism after irradiation of extracranial tumors can occur; and [3[ primary hypothyroidism may result from irradiation of regional neck nodes.