SUMMARYSeventy-one of 78 patients presenting to routine rheumatology clinics with seronegative arthritis were found to possess the histocompatibility antigen HL-A 27. Four principal components to their arthritis emerged, namely: spondylitis, sacroiliitis, peripheral polyarthritis, oligoarthritis (involvement of one or two large joints). It is suggested that each of these may be influenced by separate genetic determinants. The various combinations in which these occur warrant a unified nomenclature.A small subgroup of patients with HL-A 27-associated arthritis is described in whom the diseases may be especially benign.