Objective To discuss the clinical features and treatment strategy of patients with fulminant type 1 diabetes mellitus (FT1 DM).Methods Clinical and laboratory data of 4 patients diagnosed as FT1 DM in our hospital from January 2008 to February 2016 were retrospectively analyzed.Results 2 females and 2 males aged from 22 to 68 years old were included.They were developed to ketoacidosis rapidly after the appearance of diabetic symptoms in 2-6 days.Some cases complicated with shock,acute renal insufficiency and rhabdomyolysis.One case with late pregnancy had a stillbirth.The plasma glucose level was ranged from 35.6 to 67.8 mmol/L,HbA1c ranged from 5.8 % to 6.7%,fasting C peptide ranged from lower than 0.01 to 0.11 ng/mL and 2 -hour postprandial C peptide ranged from lower than 0.01 to 0.12 ng/mL.No improvement in C peptide levels were followed up for 9 -43 months.After acute phase,intensive insulin therapy was used to control blood glucose.The patients had a great glycaemia excursion and were subjected to hypoglycaemia.Conclusion FTlDM is a special type of diabetes developed rapidly with dangerous prognosis.It is often shown as ketoacidosis and the HbA1 c level is normal or mildly elevated.Serious cases are associated with other organ damage,pregnancy fetal death rate is very high.Because the serious impaired islet B -cell function,FTlDM patients has a great glycaemia excursion,continuous subcutaneous insulin infusion is the first choice to control plasma glycemia.