We sought to validate and extend the findings of a 282 psoriatic arthritis patient cohort from Dublin using a 219 patient cohort from Bath. The central finding of this study was that several structurally unrelated HLA alleles, including B*08:01:01, B*18:01:01, B*27:05:02, B*55:01:01 and C*06:02:01, were found to be significantly associated with particular phenotypic features of psoriatic arthritis, implying that the clinical diagnosis of psoriatic arthritis designates a genetically heterogeneous subset of individuals. Radiographic sacroiliitis was associated with either B*08:01:01, or B*27:05:02 with implications about the role of MHC molecules in an adaptive immune response. There are also implications for psoriatic arthritis diagnostic criteria since some disease features used in the criteria are under genetic control. These findings have important implications for understanding the role of MHC alleles in directing the adaptive immune response to mediate the inflammation responsible for psoriatic arthritis. (C) 2016 Elsevier Inc. All rights reserved.