Chronic nonsuppurative arthritis developed in carpal and tarsal joints of border Leicester sheep that were naturally or experimentally infected with ovine progressive pneumonia (OPP) virus. Clinical signs of arthritis began at 1 to 6.2 years of age (1 to 5.7 years after inoculation of OPP virus) and slowly progressed in severity until the sheep died or were killed. The joint lesions were characterized by edema, hyperemia, hyperplasia, and necrosis of the synovial membrane; by necrosis and erosion of articular bone; by necrosis and fibrosis of subchondral bone; and by extensive periarticular fibrosis. The OPP virus alone was isolated from arthritic joints. In 7 of 14 sheep, the arthritis was associated with interstitial pneumonitis induced by the virus. Therefore, it was concluded that OPP virus was the cause of arthritis in this group of sheep.