The prevalence of histocompatibility antigen HLA-Cw6 was 72.7% in twenty-two patients with the guttate form and 45.9% in thirty-seven patients with the vulgaris form of psoriasis; the difference between these figures and the prevalence (7.4%) in 462 healthy blood donors in Finland is highly significant. The previously observed excess of several HLA-B locus genes might be secondary to their close linkage with the Cw6 gene. Hereditary factors appear more important in the guttate than in the vulgaris form of psoriasis.