Phlegmonous esophagogastritis is a rare bacterial infection that has been reported to result in mortality. The pathophysiology of phlegmonous gastrointestinal infection is unclear, but some predisposing factors are reported. Those include immunocompromised status, alcohol abuse, malignancy and uncontrolled diabetes mellitus. We report two cases of phlegmonous esophagogastritis with newly diagnosed diabetes mellitus. A 26-year-old woman and a 56-year-old woman individually visited our hospital for sore throat, neck pain and fever. The laboratory findings of both patients demonstrated leukocytosis, and elevated serum glucose levels. HbA1c of both patients was above 11%. Enhanced computed tomography of young woman showed submucosal edema with intramural abscess along the esophagus and stomach, and that of older woman showed the same defined to esophagus. In both cases, empirical antibiotic therapy with intravenous third generation cephalosporin and metronidazole were started. Later, we identified Klebsiella pneumonia through pus culture in both cases. The symptoms of case 1 improved with conservative management with antibiotics only. However, case 2 required surgical drainage and esophagectomy. Early radiologic diagnosis of this disease and accurate identification of pathogens are important factors for good prognosis. Therefore, we emphasize suspicion of such a rare disease is needed, especially when the patient has risk factors such as diabetes mellitus.