Psoriasis and psoriatic arthritis are diseases of unknown aetiology characterized by chronic immune-mediated lesions in the skin and synovial joints respectively. The lesions are remarkably similar in appearance and in functional terms. The main differences reside in the fact, first, that the main antigen-presenting cell in the skin is the Langerhans' cell while in the joint it is probably the dendritic antigen-presenting cell and, second, that the main mesenchymal cell in the skin is the keratinocyte while in the joint it is the synoviocyte. Whether these differences merely reflect tissue-specific characteristics or are important in aetiopathogenesis is not known at present. However, the similarity in pathogenesis does mean that similar immunotherapeutic approaches can be used for their treatment.