Abstract
Rheumatoid arthritis and ankylosing spondylitis are often difficult to differentiate, though it is important to do so as the natural history and treatment of the two conditions differ. Nine patients have recently been seen, each of whom fulfilled the criteria for both rheumatoid arthritis and ankylosing spondylitis. In eight of the nine patients the histocompatibility antigen HLA-27 was present. A possible explanation of these cases is that one of the diseases occurred by chance in patients already suffering from the other, but this is extremely unlikely. If a chance association is not the correct explanation the basic concepts defining rheumatoid arthritis and ankylosing spondylitis must be reconsidered.