Osteoarthritis may be divided into primary generalized and secondary forms. Primary generalized osteoarthritis is characterized by narrowing of cartilage, marginal osteophytes, and absence of erosions. The most common sites of involvement are the distal interphalangeal joints of the fingers and the first carpometacarpal joint. Secondary osteoarthritis also results in narrowing of cartilage in the absence of erosions, but in regions of mechanical stress. Erosive osteoarthritis affects predominantly the proximal and distal interphalangeal joints, and evolves into bony fusion in 12 to 15 per cent of cases, about the same percentage of interphalangeal bony fusion that occurs in psoriatic arthritis. Ankylosing spondylitis predominates in the axial skeleton where it eventually leads to fusion of the vertebrae and sacroiliac joints. Psoriatic arthritis combines many features of rheumatoid arthritis, in which synovial inflammation predominates, and ankylosing spondylitis, in which ligamentous inflammation predominates. The hands and feet are involved to an equal extent, and in 20 per cent of patients the disorder also involves the sacroiliac joints and spine. Reiter's disease, like psoriatic arthritis, differs from ankylosing spondylitis in its inconstant involvement of the spine and greater involvement of peripheral joints. Reiter's disease differs from psoriatic arthritis in its predominant involvement of the lower limbs, particularly the feet, with relative sparing of the hands and wrists. Multicentric reticulohistiocytosis is a rare disorder in which polyarthritis usually precedes the onset of nodular cutaneous eruptions, a fact that emphasizes the importance of early roentgenologic recognition. The interphalangeal joints are the predominant sites of involvement in the hands, but eventually all of the synovium lined joints become affected, with arthritis mutilans the end result in one third of cases. The erosions are strikingly symmetrical and well circumscribed, and accompanying osteoporosis is disproportionately mild. Progressive systemic sclerosis is characterized by atrophy and dystrophic calcifications in the soft tissues, ultimately leading to joint deformities and resorption of the terminal tufts of the phalanges. Resorption of bone occurs at other sites as well, and marginal erosions may develop in the metacarpophalangeal and interphalangeal joints of the hands.