We herein report a rare case of fulminant type 1 diabetes mellitus (FT1DM) following acute pancreatitis and hypoglycemia, in which the pancreas was evaluated by serial computed tomography (CT). A 30-year-old male presented to a local hospital with a two-day history of abdominal pain and was diagnosed with acute pancreatitis based on elevated serum amylase and peripancreatic fluid collection on CT images. The patient developed sudden hypoglycemia (plasma glucose, 45mg/dL;serum C-peptide, 3.4ng/mL) the next day and hyperglycemia (plasma glucose, 250-480mg/dL) on admission day four. CT revealed a low attenuation area extending from the pancreatic head to the pancreatic tail. On admission day eight, he was referred to our hospital and diagnosed with FT1DM after he developed ketoacidosis immediately after hospitalization, with a plasma glucose level of 442mg/dL, hemoglobin A1c concentration of 5.7% and undetectable urinary C-peptide with a serum C-peptide level of 0.1ng/mL before and after intravenous glucagon loading. CT imaging revealed dramatic improvement at the time, and no pancreatic islets were detected in the pancreatic biopsy specimens.