This supplement is the first collaborative publication of members of the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA). Many of the articles are based on presentations given at the first GRAPPA meeting in August 2003, as well as at subsequent GRAPPA meetings. We thank all the contributors for a job well done. We believe that these articles highlight current key findings and review key research issues in the field of psoriasis and psoriatic arthritis (PsA) and will serve as a background for future research.

The supplement began with the difficulties in classification of PsA. Despite these difficulties, there is light at the end of the tunnel—classification criteria are currently being derived from an international collaborative effort chaired by Philip Helliwell of Leeds, England. An important point for discussion is whether PsA …