In psoriasis vulgaris, the HLA class I Cw6 specificity has previously been recognized as the most commonly associated antigen serologically. This study was carried out to investigate whether or not the gene controlling the susceptibility to psoriasis vulgaris existed on the HLA, especially the HLA-C region. At first, we analyzed the restriction fragment length polymorphism (RFLP) of 13 patients with psoriasis vulgaris and 6 healthy controls who were all positive for at least one allele of HLA-Cw6. To characterize RFLP in psoriasis patients who did not have HLA-Cw6, 12 patients and 10 healthy controls who had HLA-Cw7 were also examined. Southern hybridization of genomic DNA demonstrated that DNA polymorphisms of the HLA-C antigen gene could not be found in any psoriasis vulgaris patient whether HLA-Cw6 or Cw7. However, a 4.5 kb BamHI fragment and a 3.1 kb PstI fragment were lacking in some healthy controls who had either HLA-Cw6 or Cw7. This study suggests that the presence of RFLP in the HLA-C gene is associated with psoriasis vulgaris. These specific fragments may help predispose individuals to psoriasis vulgaris, or may be essential for the development of the disease.