Ankylosing spondylitis, Reiter's syndrome, psoriatic arthritis, and arthritis of inflammatory bowel disease.

These diseases are classified together as the seronegative spondyloarthritides. They are characterized by an association with the cell surface antigen HLA-B27, sacroilitis and spondylitis, inflammatory peripheral arthritis, enthesopathy (abnormalities in ligamentous attachments), and the absence of rheumatoid factor. Extra-articular complications are common. The diseases are usually treated with nonsteroidal anti-inflammatory drugs and physical therapy.