Association of HLA-Cw6 with psoriasis vulgaris (PV) is well known. Three amino acids, aspartic acid at position 9 (9Asp), alanine at position 73, and aspartic acid at position 90, which are found in HLA-Cw6, are hypothesized to play a role in susceptibility to PV. Among these amino acids, 9Asp located on B pocket of HLA molecule, which is the binding site of anchor amino acid in antigen peptides. Therefore, the existence of 9Asp in HLA-C locus is considered to influent the antigen binding. In order to know the role of HLA-Cw6 on the pathogenesis of PV, the incidence of 9Asp in HLA were analysed in 50 patients with PV by amplifying 9Asp-specific DNA fragment with PCR technique. As a result, the incidence of 9Asp in PV patients was not significantly higher than that in healthy individuals. In addition, the existence of 9Asp was associated with neither clinical severity, nail deformity, arthralgia nor age onset of the disease. This result supports the hypothesis that HLA-C locus is just linked to the gene controlling susceptibility to PV.