Since the first case was contracted by coronavirus disease-19 (COVID-19) in Daegu, Korea in February 2020, about 6,800 cases and 130 deaths have been reported on April 9, 2020. Recent studies have reported that patients with diabetes showed higher mortality and they had a worse prognosis than the group without diabetes. In poorly controlled patients with diabetes, acute hyperglycemic crises such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS) also might be precipitated by COVID-19. Thus, intensive monitoring and aggressive supportive care should be needed to inadequately controlled patients with diabetes and COVID-19 infection. Here, we report two cases of severe COVID-19 patients with acute hyperglycemic crises in Korea.