A 72-year-old woman was admitted to the hospital because of hypoglycemia. Her glucose level on admission was 40 mg/dl. A giant intrathoracic mass was demonstrated on X-ray, Computerized tomographic scan of the cerebrum showed no abnormalities. Serum insulin and C-peptide levels were within normal limits. The mass (weight 2.3 kg) and the right lower lobe were excised by thoracotomy and the hypoglycemia ceased. Pathological examination revealed the mass to be a localized fibrous tumor of the pleura.