Epidemiologic, clinical, radiologic and serologic evidence suggests that psoriatic arthritis is a specific entity and not the coincidental occurrence of 2 common diseases, psoriasis and rheumatoid arthritis. Psoriatic arthritis may be defined as psoriasis associated with inflammatory arthritis (peripheral arthritis or spondylitis or both) and usually a negative serologic test for rheumatoid factor. Clinical characteristics of the disease include: almost equal distribution between males and females; peripheral arthritis involving only a few small joints in asymmetical fashion; involvement of distal interphalangeal joints; sausage digits; arthritis mutilans; ankylosing spondylitis; goutlike onset; and higher frequency of nail involvement than occurs in uncomplicated psoriasis. The rash may present with arthritis, or, equally, may precede or succeed joint involvement. With regard to pain and disability, the prognosis in psoriatic arthritis is better than in rheumatoid arthritis.