Eighteen cases of long-standing rheumatoid arthritis and superimposed pyarthrosis were reviewed to determine the most distinguishing radiographic features. Soft-tissue changes allowed earlier diagnosis in the knee and ankle joints and consisted of large asymmetrical joint effusion and fat-pad edema. Bony articular changes were more helpful than soft-tissue changes in the wrist and hip because of the paucity of adjacent extracapsular fat and were associated with delayed radiographic recognition of superimposed pyarthrosis. These changes are presumptive evidence of complicating septic arthritis and their presence necessitates needle aspiration and culture of the joint for proper definitive treatment.