Synovial cysts of the shoulder are rare. The case described illustrates the particular problems encountered at this site. As at other joints, these cysts appear during arthropathies of various types: degenerative or inflammatory (rheumatoid arthritis). They are often associated with rupture of the rotator cuff. Their essential characteristic is an expansive and recurrent tendency which leads to striking giant swellings, capable of causing vascular compression. Treatment involves surgical excision. This suffices when the cyst does not communicate with the joint cavity. In contrast, when there is a breach in the capsule through which the cyst derives its synovial fluid supply, repair is necessary to prevent recurrence.