The pathogenesis and pathology of 421 cases of shoulder ulcerations in sows were examined and compared with 95 normal shoulders. The ulcerations started on the surface as a result of thrombosis and gradually progressed inwards to the tuber spina scapula. At each stage the lesions contained multiple bacterial colonies, and as the ulcerations progressed the amount of granulation tissues and fibroplasia increased. In 45 of the 95 normal shoulders there were areas of acute haemorrhage, oedema and fibroplasia, which followed the cords of normal fibrous tissue. However, in these cases the tissue was never necrotic, and the reaction was comparable with the subcutaneous lesions caused by blunt trauma to the skin.