Insulin autoimmune syndrome (IAS) is characterized by spontaneous hypoglycemia with extremely high insulin levels and the presence of circulating autoantibodies against insulin, in patients who have never been exposed to exogenous insulin. We report two patients with the syndrome. A 36 years old male presenting with hypoglycemia in the emergency room had an oral glucose tolerance test showed basal and 120 min glucose levels of 88 and 185 mg/dl. The basal and 120 min insulin levels were 2759 and 5942 ?UI/ml. The presence of an insulin secreting tumor was discarded. Anti-insulin antibodies were positive. He was successfully treated with a diet restricted in carbohydrates and frequent meals in small quantities. A 65 years old female presenting with hypoglycemia in the emergency room had the fasting insulin levels of 1910 µUI/ml. No insulin secreting tumor was detected by images and anti-insulin antibodies were positive. The polyethylene glycol precipitation test showed a basal and after exposition insulin level 1483 and 114 µUI/ml, respectively. She responded partially to diet and acarbose and required the use of prednisone with a good clinical response.Insulin autoimmune syndrome (IAS) is characterized by spontaneous hypoglycemia with extremely high insulin levels and the presence of circulating autoantibodies against insulin, in patients who have never been exposed to exogenous insulin. We report two patients with the syndrome. A 36 years old male presenting with hypoglycemia in the emergency room had an oral glucose tolerance test showed basal and 120 min glucose levels of 88 and 185 mg/dl. The basal and 120 min insulin levels were 2759 and 5942 ?UI/ml. The presence of an insulin secreting tumor was discarded. Anti-insulin antibodies were positive. He was successfully treated with a diet restricted in carbohydrates and frequent meals in small quantities. A 65 years old female presenting with hypoglycemia in the emergency room had the fasting insulin levels of 1910 µUI/ml. No insulin secreting tumor was detected by images and anti-insulin antibodies were positive. The polyethylene glycol precipitation test showed a basal and after exposition insulin level 1483 and 114 µUI/ml, respectively. She responded partially to diet and acarbose and required the use of prednisone with a good clinical response.