The recovery of the hypothalamus-pituitary-adrenal axis after high-dose short-term steroid treatment was investigated in 18 patients undergoing stereotactic thalamotomy during local anaesthesia, in whom perioperative betamethasone was given in a high dose for 4-5 days and withdrawn without tapering. There were no untoward clinical effects of the abrupt withdrawal of the steroid. Seven patients undergoing the same operation without steroid treatment served as controls. The recovery of the hypothalamus-pituitary-adrenal axis after steroid treatment was studied by basal plasma cortisol levels. In 12 patients the reserve capacity of the system was investigated by the cortisol response to insulin induced hypoglycemia as well. It was found that the basal plasma cortisol levels had normalized 2 days after the abrupt withdrawal of betamethasone. In spite of normalized basal cortisol levels the capacity to respond to stress was reduced for a further 3 days. Six to seven days after the steroid withdrawal the stress response was, however, completely restored. It is concluded that high dose, short term steroid treatment can be withdrawn abruptly, omitting the usual tapering of the dose. During the first week after steroid withdrawal the cortisol response to stress is reduced even in the presence of normal basal cortisol levels. During this period extra corticosteroids must be supplied in case of an emergency situation or an operation.