Dumping syndrome is a complication of gastric surgery including bariatric surgery, and which is induced by rapid gastric emptying and increased intestinal motility. We should note that hypoglycaemia due to dumping syndrome can occur in patients with type 2 diabetes. However, the pathogenesis of dumping syndrome in patients with type 2 diabetes is not fully investigated. We investigated the changes in plasma glucose, serum insulin, plasma glucagon, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) by the 75g oral glucose tolerance test (OGTT) in 3 patients with and without type 2 diabetes who had gastric surgery. Significant hyperinsulinemia was observed in non-diabetic patients, but not in a diabetic patient. On the other hand, plasma GLP-1 levels significantly increased after glucose intake in a diabetic patient. Increased secretion of GLP-1 may have caused reactive hypoglycaemia in patients with type 2 diabetes undergoing gastric surgery.