Six patients with rheumatoid arthritis are described in whom giant synovial cysts of the popliteal space and calf developed. In some, these cysts extended to the level of the Achilles tendon. All complained of calf pain and were found to have swelling and tenderness in the region. In three patients, the onset of symptoms was relatively acute and suggested the diagnosis of thrombophlebitis. In two others, a softtissue tumor was suspected. The proper diagnosis was established by means of contrast arthrography of the knee. These massive cysts, measuring up to twenty-eight centimeters in length, were surgically excised, and all were found to communicate with the posterior part of the knee joint, arising at the level of the femoral condyle in close proximity to the semimembranosus and the medial head of the gastroenemius. The walls of these cysts were thick, fibrotic, and densely infiltrated with chronic inflammatory cells, chiefly lymphocytes and plasma cells. The lining of the cyst appeared synovial in character, and the inflammatory reaction resembled that found in rheumatoid arthritis. The origin of these cysts remains controversial. Surgical excision appears to be the treatment of choice.

The findings in these six patients are compared and added to those of twentythree other cases of giant (rheumatoid) synovial cysts of the calf previously reported. These symptoms are so frequently misdiagnosed as thrombophlebitis that contrast arthrography of the knee should be considered in any individual with rheumatoid arthritis who has calf pain and swelling.