Genetic polymorphisms of the serum complement components (C2, C4, C6, and C7) and HLA-antigens were studied in Japanese patients with psoriasis vulgaris. Among these complement components, positive associations with C4A4 and C4B2 were obtained. In addition to the previously reported haplotypes HLA-Cw6-B13 and Cw6-Bw57-DR7, the extended haplotype HLA-Cw11-Bw46-C4A4. 4B2-BFS-C2C-DRw8 may be a high risk haplotype in Japanese patients with psoriasis vulgaris.