Histocompatibility antigens were determined in 93 unrelated Irish psoriatic patients. The results were compared with the HLA profiles of 253 healthy unrelated Irish controls. There was a statistically significant increase in HLA B17 (P < 0.0001), B13 (P = 0.02), and B27 (P = 0.015) among the psoriatic patients. The frequency of HLAB8 (P = 0.002) and HLA B12 (P = 0.001) was lower than expected. When the subgroup of patients with psoriatic arthritis was evaluated separately, it was noted that the increase in HLAB27 was highly statistically significant (P < 0.0001). The relative risks of an Irish patient carrying the major psoriasis associated alleles of developing psoriasis are documented;-possession of the HLA B17 and B13 antigens conferring a risk of developing psoriasis vulgaris of 6.08 and 2.9 respectively. Further values for psoriatic subsets are presented. This is the first report of HLA typing in an Irish psoriatic population. The findings are discussed and compared with other population groups.