HL-A antigen B17, associated with a high familial frequency of psoriasis, is significantly correlated (r = 0.8242) with psoriasis case rates in several countries but HL-A antigen B13 alone, associated with ‘early age onset’ cases of psoriasis, is not significantly correlated (r = 0.4299) with case rate. A sum of the B13 and B17 antigen frequencies, since either antigen has been associated to psoriasis, is correlated with case rate in the populations surveyed (r = 0.7405) and is statistically signific