Interstitial irradiation using yttrium-90 (90Y) rods implanted by needle into the pituitary gland was used as primary treatment in 16 patients with pituitary dependent Cushing's disease. Clinical and biochemical remission was observed within three or six months in 13 and in the remaining three after a supplementary implant. There was no perioperative morbidity. Follow-up from the time of definitive operation ranged from six to 123 months (mean 39). No recurrence has been observed. The return of a normal diurnal cortisol rhythm has been observed in 10/12 patients studied after remission. Some form of long-term pituitary hormone replacement therapy was required in only the six patients who had received the largest irradiation dose. Implantation of 90Y is safe and effective treatment for patients with Cushing's disease, comparing favourably with selective trans-sphenoidal pituitary surgery.