From the Departments of Medicine and Dermatology, Royal Liverpool Hospital, Liverpool L69 3BX.

SUMMARY HLA phenotypes were determined in 50 patients with psoriasis alone and in 50 patients with psoriasis and psoriatic arthritis. Positive associations were found in both groups with B13, B17, B37, Cw6, and DR7, and in addition with C4A6. Higher relative risks were found in respect to the patients with psoriasis alone compared with those with arthritis, and this suggests the involvement of additional genetic factors predisposing to peripheral arthritis. In patients with psoriasis only, the presence of Cw6 was associated with a significantly earlier age of onset. Analysis of the pattern of familial aggregation of psoriasis suggested the likelihood that several gene-tic loci might be involved in producing increased susceptibility. ' 2 That HLA linked loci are involved was shown by the demonstration of positive associa-tions between psoriasis and several HLA antigens, and in particular with B17, B13, Cw6, and DR7.39 Evidence for familial aggregation of psoriatic arthritisl and for the association of the presence of arthritis with HLA-B27"1 and possibly other HLA antigens'2 suggested that additional genetic factors might predispose to the development of joint disease. The present study was aimed at examining further the relationship between the HLA-A, B, C, and DR polymorphisms in groups of patients with psoriasis only or with psoriasis and peripheral psoriatic arthritis, together with a study of the C4 polymorph-ism (the C4A and C4B loci being closed linked to the HLA-B and DR loci). Evidence has previously been put forward for heterogeneity in relation to age of onset of psoriasis,4 "1 and this was looked at again in the light of the more recently discovered associa-tions. Patients and methods Fifty successive patients with psoriasis vulgaris but without evidence of peripheral psoriatic arthritis were ascertained in a dermatological outpatient department. A further 50 patients with psoriasis and with a chronic peripheral arthritis judged to be of a clinical pattern known to be associated with psor-iasis (groups 1, 2, 3, and 4 in a previous study of psoriatic arthritis") were ascertained in a rheumato-logical unit. Control subjects were laboratory and nursing staf