1. Clinical, radiological and pathological findings in seven cases of rheumatoid arthritis with giant bone cysts are presented.

2. In three cases the large cysts represented massive involvement of subchondral bone by rheumatoid granulation tissue; in two cases the cysts were due to rheumatoid nodules, and in two other cases the diagnosis was made only on the radiological findings.

3. The pathogenesis of this condition is discussed. It is suggested that cystic bone lesions in rheumatoid arthritis can arise either by replacement of subchondral bone by the rheumatoid process, or by the formation of rheumatoid nodules within the bone.