HL‐A typing was performed on 110 psoriatics and confirmed the highly increased frequencies of W17, HL‐A13 and HL‐A1‐W17 haplotype previously reported in two American works. Psoriasis was found twice as frequently among close relatives of W17 patients as in other groups. Yet, psoriasis and W17, when associated in individuals, were not always transmitted together genetically. Articular signs did not seem to have any relation to HL‐A antigens. The comparison of our results with those of two previous investigations showed probable modifications in the distribution of several HL‐A antigens: increased HL‐A1 and HL‐A2, decreased HL‐A9, W28, HL‐A7 and HL‐A12.Some immunologic disturbances appeared particularly frequently in psoriasis, namely, an elevated IgE titer in serum and an anti‐IgG activity demonstrated on peripheral blood lymphocytes and in serum. It has been noted that numerous diseases related to HL‐A are accompanied by immune perturbations.The mechanisms of association between HL‐A, immunity disorders and some diseases are discussed, including the following possibilities: indirect action of HL‐A through an epistasis phenomenon; direct action of HL‐A by antigenic cross‐reactions favouring the penetration of an aetiologic agent; no action of HL‐A which would thus only be a genetic marker linked with a lo