A distinctive peripheral arthropathy associated with psoriasis is well recognized, the classical pattern describing an asymmetrical oligoarthritis with predominant distal interphalangeal joint (DIP) involvement. There is some dispute about the frequency of this classical pattern and of the pattern of symmetrical polyarthritis resembling rheumatoid arthritis. Some of the dispute may be a result of loose definitions. In this clinical and scintigraphic study of 50 patients with psoriatic arthritis we have used tighter definitions of disease pattern and have found that 68% of this group had a symmetrical polyarthritis similar to that found in rheumatoid arthritis. Yet, arthritis associated with psoriasis differs from rheumatoid arthritis in a number of ways including the pattern of joint involvement, extra-articular osseous features, and radiological changes. Scintigraphic changes in psoriatic arthritis suggest that subclinical involvement of the manubriosternal and sternoclavicular joints is common, suggesting an association between psoriasis vulgaris and arthro-osteitis. A modified classification of the osteoarticular manifestations of psoriasis is proposed.