In a study on genetic markers (22 HLA antigens, 6 blood group types, 3 serum groups and 3 red cell enzymes) in psoriatic patients the following statistically significant differences were found between patients and controls: 1) The HLA factors B13 and Bw17, the Lewis blood group Le(a-b-) and the blood group factor A were overrepresented among the patients. A decrease in the frequencies of the HLA factors B7 and B8 was noted, confirming several similar findings by other investigators. 2) In the Ss (MNSs), C3 and haptoglobin (Hp) systems the psoriatic patients showed a decreased frequency of heterozygotes. 3) Among patients with continuous symptoms the blood group factor A and the blood group Fy (a+b-) were overrepresented. 4) Patients with late onset of psoriasis showed an increase in the blood group (Fy(a+bb-) and a decrease in the haptoglobin type 2-2. Relative risk figures have been calculated for different phenotypes. Certain genetic combinations were found to be grossly overrepresented among the patients. The mechanisms behind the associations appear to differ. In three systems (Ss, C3 and Hp) the deviations in psoriatic patients suggest the presence of heterozygote advantage.