OBJECTIVE: Diagnostic criteria in fulminant type 1 diabetes, a novel subtype of type 1 diabetes, remain unclear. RESEARCH DESIGN AND METHODS: We analyzed basal and longitudinal changes of serum C-peptide levels during a 75-g oral glucose tolerance test (OGTT) in 125 consecutively recruited patients with type 1 diabetes including fulminant type 1 diabetes (n = 25) and acute-onset type 1 diabetes (n = 100). Discriminating criteria of fulminant type 1 diabetes were examined using receiver-operating characteristic curve analysis and multiple logistic regression analysis. RESULTS: The integrated values of serum C-peptide response during OGTT (SigmaC-peptide) in fulminant type 1 diabetes at onset, 1 year, and 2 years after onset were markedly lower than those in acute-onset type 1 diabetes. None of the patients with fulminant type 1 diabetes had improvement of C-peptide response to OGTT. Fasting C-peptide values at onset in fulminant type 1 diabetes were significantly lower than those in acute-onset type 1 diabetes. We established diagnostic criteria of serum C-peptide and HbA(1c) levels at onset that discriminate fulminant type 1 diabetes from acute-onset type 1 diabetes with high sensitivity and specificity: a criterion in which the levels of both the fasting C-peptide is <or=0.033 nmol/l and HbA(1c) is <or=8.0% or a criterion in which the levels of both the SigmaC-peptide is <or=0.540 nmol/l and HbA(1c) is <or=8.0%. CONCLUSIONS: Fulminant type 1 diabetes has extremely low beta-cell function at onset that rarely recovers after onset. Sensitive and specific diagnostic criteria were established for detection of fulminant type 1 diabetes based on serum C-peptide and HbA(1c) levels at onset.