Computed tomographic arthrography (CTA) of the shoulder is currently the best investigation of the unstable shoulder. 47 patients had CTA in the prone oblique position to assess its ability to demonstrate both anterior and posterior capsular mechanisms simultaneously. The first five patients were also scanned supine oblique to allow direct comparison with the prone oblique position. All studies were reviewed retrospectively by three musculoskeletal radiologists with regard to how well the relevant structures were demonstrated. In the prone oblique position, the anterior capsule was well shown in 98%, anterior labrum 98%, posterior capsule 91%, posterior labrum 89%, subscapularis tendon 98%, biceps tendon 100% and biceps tendon proximal insertion 78%. In 86% of cases both anterior and posterior structures were well seen simultaneously. In the five cases that also had supine imaging the prone oblique images were superior. The cause of poor demonstration of structures was invariably insufficient intraarticular air. It is concluded that the prone oblique is an excellent technique for CTA of the shoulder and should become the standard position for assessing shoulder instability.