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The term spondyloarthropathy (SpA) describes and defines a group of related inflammatory joint disease that share characteristic clinical features and a unique association with the major histocompatibility complex class I molecule HLA-B27. Five subgroups can be differentiated: ankylosing spondylitis, reactive arthritis, psoriatic arthritis, arthritis associated with inflammatory bowel disease, and undifferentiated SpA. The sacroiliac joints are centrally involved in the SpA, most clearly and pathognomonic in ankylosing spondylitis, in which most patients are affected early in the disease. Overcoming some of the diagnostic difficulties of early sacroiliitis, dynamic magnetic resonance imaging was shown to visualize both acute and chronic changes in the sacroiliac joints. The inflammation in the sacroiliac joints in patients with SpA was recently examined in more detail; using immunohistology and in situ hybridrization, T cells, macrophages, and various cytokines were found in infiltrates. Biopsy specimens were obtained under guided computed tomography, and in the same study, intra-articular corticosteroid treatment was successfully undertaken. Further investigation of such biopsy specimens showed the absence of DNA of reactive arthritis-associated bacteria. The pathogenesis of the SpA and the reason for the tropism for the sacroiliac joints is still obscure. The nature of the relation of the genetic background of SpA to initially triggering bacterial infections remains to be established. In chronic disease, autoimmune mechanisms might be more important.