Using a DNA probe for the switch region of immunoglobulin heavy chain genes, together with the restriction endonuclease Sst I, we detected a particular polymorphic DNA pattern in 17 of 28 patients (60.7%) with psoriatic arthropathy but in only 5 of 41 patients (12.2%) with psoriasis alone. Our findings suggest that genes in the immunoglobulin region confer susceptibility to the development of arthropathy in patients with psoriasis.