The endocrine status of 106 patients with undifferentiated small cell carcinoma of the lung was evaluated before treatment was begun. Almost one half of the patients had evidence of abnormal control of the secretion of adrenal cortical steroids, manifested by loss of diurnal rhythmicity or dexamethasone suppressibility. Only two had the clinical syndrome of ectopic ACTH secretion. Evidence of inappropriate secretion of vasopressin was found in 38% of the patients, most of whom also had abnormalities of corticosteroid secretory pattern. About one half of the patients had evidence of abnormal glucose tolerance, and many also had a paradoxical rise of plasma growth hormone concentration after glucose administration. The levels of the other hormones studies were normal. The pattern of hormone abnormality observed in these patients appears to be relatively specific for small cell undifferentiated carcinoma, and is different from that observed in other pulmonary tumors. Patients with abnormal control of plasma cortisol had a worse prognosis than those with normal adrenal function, largely because of decreased response rates to chemotherapy. Other endocrine abnormalities were of no prognostic significance.