A case of insulin autoimmune syndrome (IAS) is reported and the related literature reviewed. A 49 year-old male patient with Graves' disease had been on tapazole therapy for one month. There was no history of diabetes hypoglycemia and previous administration of insulin. On the morning of 21 August, 1984, at 10 a. m. the patient presented an acute episode of hypoglycemia with plasma glucose dropped to 16 mg/dl and was relieved with I.V. glucose. Clinical investigations revealed fasting hypoglycemia, impaired glucose tolerance and postprandial hypoglucomia. There was a large amount of spontaneously occurred insulin antibody in the serum of the patient. After pretreatment with PEG, the measurements of plasma free insulin revealed fasting hyperinsulinemia and lack of insulin release peak after glucose loading. Immunological studies showed the presence of autoantibodies against insulin, TSH receptors, thyroid microsomes and pituitary cells. One month later the fasting hypoglycemia disappeared, the glucose tolerance test became normal and the amount of insulin autoantiboy markedly decreased.