In 101 white psoriatic patients, two histocompatibility (HL-A) specificities were significantly altered from expected values. The levels of W16 and W17 were found to be substantially increased, suggesting that persons with these antigens are at increased risk of having psoriasis. Clinically distinct patient groups were also observed. Antigens W16 or W17 or both were more prevalent in psoriatic patients who had extensive disease involvement, and patients with W17 antigen had an earlier age of onset as compared to patients with W16 antigen. In one family in this study, a linkage between psoriasis and a specific HL-A haplotype was also observed, further supporting the concept that the HL-A system may serve as a marker for genes affecting specific disease susceptibility.