We performed HLA-A, -B, and -C antigen and -DR DNA typing in 111 Japanese patients with idiopathic thrombocytopenic purpura (ITP). DRB1*0410 was significantly increased in ITP patients compared with healthy controls (relative risk = 9.52, P < .05), but the other DRB1*04 alleles showed no significant differences. On HLA-DR serotyping, patients with Vogt-Koyanagi-Harada disease (VKH) had a high frequency of DR4, so we compared the frequencies of DRB1*04 suballeles between ITP and VKH. The high frequency of DRB1*04 was dependent on DRB1*0405 in VKH, but on DRB1*0410 in ITP. Plasma autoantibodies were studied in 111 patients using a microtiter well assay. Thirty-six patients had anti-GPIIb/IIIa autoantibodies, and antibody positivity was associated with HLA-DR4 (29 of 36, 80.6% v 28 of 75, 37.3%) but not with DRB1*0410. When HLA-DR4 and DRB1*0410 were compared between patients with a good or poor response to prednisolone, HLA-DR4 was decreased and DRB1*0410 was significantly decreased (chi2 = 11.455, P < .01) in patients with a good response. In conclusion, this study showed that genetically determined factors influence the course of ITP. However, our findings should be considered preliminary because of possible racial differences in HLA status between Japanese and other ITP patients.