OBJECTIVE—Some type 1 diabetic patients have a distinct phenotype characterized by the absence of pancreatic autoantibodies and fulminant clinical symptoms at onset, including marked hyperglycemia, severe diabetic ketoacidosis, and normal to near-normal HbA1c levels with complete destruction of β-cells. However, little is known about genetic factors of this distinct subtype of diabetes (fulminant autoantibody-negative type 1 diabetes). RESEARCH DESIGN AND METHODS—We analyzed HLA-DQ genotypes in fulminant autoantibody-negative type 1 diabetes (n = 22) and autoantibody-positive type 1 diabetes (immune-mediated type 1 diabetes, n = 78) recruited from a cohort between 1980 and 2000. RESULTS—Fulminant autoantibody-negative type 1 diabetes had a significantly high prevalence of the HLA-DQA1*0303-DQB1*0401 haplotype in a homozygous manner (RR 39) or in a heterozygous manner with the HLA-DQA1*0302-DQB1*0303 haplotype (RR 13). In contrast, autoantibody-positive type 1 diabetic patients had a high prevalence of the HLA-DQA1*0302-DQB1*0303 haplotype in a homozygous manner (RR 10) or in a heterozygous manner with the HLA-DQA1*0303-DQB1*0401 haplotype (RR 12). CONCLUSIONS—Pathogenic roles of genotypic combinations of specific HLA-DQ haplotypes in a homozygous manner are suggested as causative mechanisms of aggressive β-cell damage in a subtype of autoantibody-negative type 1 diabetes with fulminant clinical features.