In a study of nonsurgical therapy of Cushing's disease, 62 patients received O,p'-dichlorodiphenyldichloroethane (O,p'DDD), 16 of whom also received cobalt irradiation of the pituitary. After an initial treatment period averaging eight months, a remission of the disease was obtained in 38 of the 46 patients given O,p'DDD alone and in all patients who received drug combined with radiation. Although 60 per cent of these patients subsequently relapsed, additional courses of drug or radiation therapy were usually effective, and 63 per cent of the entire group of patients have so far been kept under control without adrenalectomy. (Forty patients have been followed for at least two years after the initial course of treatment.). O,p'DDD produced little gastrointestinal discomfort; an increased serum cholesterol was the main side effect. This drug allows long-term medical management of Cushings disease in most cases. Whether the combination of O'p'DDD with pituitary radiation is the best therapy has not been established.