SUMMARY Histocompatibility antigens were determined in 60 patients with psoriatic arthritis. The patients were divided into clinical subgroups according to axial or peripheral joint involvement, disease severity based on number of peripheral joints involved, and the presence or absence of bone erosions. The total group showed a significant increase in frequency of HLA-A1, B17, B27, and DR7 when compared with a control population. The subgroup with spondylitis had a significant increase in frequency of HLA-B27 when compared with patients with peripheral arthritis (p<0-001). The subgroup with peripheral arthritis alone had a higher frequency of HLA-DR7 than the control group (p<0.001). There were also significant associations between HLA-DR7 and chronic severe disease (p<0-001) and between HLA-DR4 and the presence of erosions (p

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SUMMARY HLA typing in Wellington revealed a stronger association of multiple sclerosis with DR2 than with DQwl. The association with DQwl appeared to be due to linkage disequilibrium of this antigen with DR2. These results, when considered in conjunction with other studies, are most easily explained by the hypothesis that susceptibility to multiple sclerosis is influenced by multiple risk factors, with DR2 being an important risk factor in Caucasoid populations.