Ever since Alibert's first description (1822),1 and since Bourdillon's thesis (1888)2 on the association of psoriasis and arthritis, there has been controversy about the nature of the arthritis. Is it an entity or a form of rheumatoid arthritis or infectious arthritis? For a possible answer to this question, we have performed clinical studies and serological tests to ascertain whether psoriatic patients with arthritis have a different disease from rheumatoid arthritis. Arthritic Findings The American Rheumatism Association (1958)3 has proposed diagnostic criteria for rheumatoid arthritis, in order to obtain more uniformity in cases so listed. For a diagnosis of classical rheumatoid arthritis, they have listed eleven characteristics, of which seven must be met. The symptoms must be continuous for at least six weeks in order to be valid. Some of the more important criteria are as follows: morning stiffness, swelling in at least one joint in an interval