Coexistence of ankylosing spondylitis and rheumatoid arthritis

Ankylosing spondylitis and rheumatoid arthritis share many common features. However the presence of rheumatoid factor, histologically classic rheumatoid nodules, and the histocompatibility cell wall antigen (HLA-B27) helps distinguish one from the other. Two cases are reported in which these features established the coexisting diagnoses of ankylosing spondylitis and rheumatoid arthritis.