Nineteen patients thought to have Cushing's disease were treated by transsphenoidal microsurgery; the type of operation performed depended upon the findings in the individual patient. Seventeen patients remitted. Failures occurred in a patient with an invasive macroadenoma and in a patient who was subsequently found to have a thymic carcinoid tumour secreting ACTH. One patient who remitted suffered a recurrence during pregnancy, 30 months after operation. The ten patients (Group I) who had a selective removal of a microadenoma or a limited resection of the gland were often GH deficient, but seven regained cortisol reserve and all ten regained normal pituitary-thyroid and pituitary-gonadal responses. By contrast abnormalities of pituitary function were common in nine patients who had a radical or total hypophysectomy. We conclude that transsphenoidal microsurgery is the best treatment for Cushing's disease and that, when feasible, a selective microadenomectomy is the most appropriate operation.