The medical records and radiographs of the hands, wrists, and feet of 50 patients with psoriatic arthritis and 34 with erosive osteoarthritis were reviewed. Polyarticular arthrutis in psoriatics is often unilateral and the joints of a single ray may be selectively affected. Their erosions ten to occur at the "bare areas" of joints, whereas in erosive osteoarthritis it is the subchondral cortex that is primarily affected. Linear periosteal bone apposition is common in the latter. The distribution of arthritis, sites of bone erosion, and character of periosteal bone apposition form distinctive patterns in these conditions which are important in differential diagnosis.