Chronic unremitting, generally symmetric, erosive polyarthritis was studied in 8 dogs. The disease had clinical, serologic, radiographic, and pathologic changes similar to those of rheumatoid arthritis of man. The condition occurred mainly in smaller breeds of dogs, with time of onset from 8 months to 8 years of age, Characteristic radiographic changes were seen in the joints several weeks to several months after the appearance of the initial lameness. Synovial fluid contained an increased number of neutrophils, and synovial fluid and synovial tissues were sterile for anaerobic and aerobic bacteria, mycoplasma, chlamydia, and viruses. Corticosteroids were therapeutically ineffective in all of the cases; however, corticosteroids, cyclophosphamide, and azathioprine were effective when used in combination in several dogs.