Analysis of clinical material obtained from the individuals (49 psoriasis patients and 357 individuals without this disease) representing three ethnic populations of Dagestan (Avars, Dargins, and Kumyks) was performed. Polymorphism of the blood group loci AB0, Rhesus (RH), Kell, P, and Lewis, as well as of the protein-encoding loci for haptoglobin (HP), group-specific component (GC), and the enzymes, including glycosylase (GL01), esterase D (ESD), 6-phosphate dehydrogenase (6PDG), and acid phosphatase (ACP), was studied. It was demonstrated that in the pooled sample of Avars and Kumyks the Lewis system phenotype Le(a-b-) and the RH homozygotes (ee/ee) were statistically significantly more frequent among the psoriasis patients (P = 0.0488 and P = 0.0166, respectively), than among healthy controls of the same ethnic groups. It was suggested that for the pooled sample of Avars and Kumyks, homozygosity for the recessive RH allele (ee/ee) in combination with the Le(a-b-) phenotype, representing homozygosity for recessive allele le, was the risk factor for the development of psoriasis.