Human leukocyte antigen-D region-related alleles (human leukocyte antigen DR and DQ) and human leukocyte antigen class I alleles were typed serologically in 31 Japanese patients with autoimmune hepatitis. These patients had increased serum levels of AST and IgG, high titers of autoantibodies, no history of blood transfusion and were negative for HBsAg and antibodies to HBc. Three hundred eighty-six healthy subjects and 30 patients with cryptogenic chronic hepatitis served as control groups. The frequency of DR4 was significantly higher in autoimmune hepatitis patients (90.3%) than in healthy subjects (38.6%) and in cryptogenic chronic hepatitis patients (30%). The frequency of Bw54 was significantly higher in autoimmune hepatitis patients (45.2%) than in healthy subjects (10.9%). The risk to DR4-positive subjects for autoimmune hepatitis was 14.8 relative to healthy subjects. Two of 31 patients (6.5%) with autoimmune hepatitis were positive for antibody to hepatitis C virus; both clearly satisfied criteria for autoimmune hepatitis and both had Bw54 and DR4. This study revealed a highly significant association of autoimmune hepatitis with human leukocyte antigen Bw54 and DR4 in Japanese patients. Among the DR4-positive patients with autoimmune hepatitis, no significant differences were seen between those positive or negative for Bw54 with regard to clinical or laboratory data, relapse of disease or efficacy of prednisolone. Thus human leukocyte antigen class II alleles contribute to susceptibility and resistance to autoimmune hepatitis in Japanese patients, with distinct racial differences from those in white patients.