AbstractThe normal subdeltoid bursa is not visible on the plain films of the shoulder. The extrasynovial fat is clearly defined, however, deep to deltoid along the outer margin of the upper end of the humerus for about 2·5 cm. It is about 1–2 mm wide. It passes up deep to the deltoid muscle and on to the under-surface of the acromion process in the form of a thin crescent. When the bursa is enlarged by synovial fluid or by synovial thickening, or both, as in rheumatoid arthritis, it can be visible on the plain radiographs. In some cases the bursa is not clearly separated from the deltoid and presents as a mass lesion.(1)Direct evidence. The bursa can be seen as a mass lesion between the humeral shaft and the deep surface of deltoid. It has a well-defined lower lateral, caudal and medial margin outlined by extrasynovial fat.(2)Indirect evidence. The bursal enlargement acts merely as a mass lesion, displacing the deltoid muscle laterally, away from the humeral shaft and causing the deltoid just distal to its origin to be thicker and rounder than usual. Normally the deltoid muscle is flattest at this point. These features are demonstrated in the line diagrams. The density of the deltoid bursal complex is increased.No previous description of the soft tissue signs of an enlarged subdeltoid bursa has been found. This finding may alert the clinician to the possibility of shoulder joint disease, particularly in relation to rheumatoid arthritis.