The purpose of this study is to evaluate the morphologic findings of the labrum and capsuloligamentous structures in the normal and unstable shoulders and to compare the effectiveness of double contrast arthrographic findings which were taken with supine and prone position. We reviewed the computerized tomographic arthrogram in 9 cases of normal shoulder and l3 cases of shoulder instability. We evaluated the shape of the labrum, the attachment of anterior capsule and the amount of capsular laxity. Capsular laxity was compared each others in normal, the unidirectional instability and the multidirectional instability. Among these materials, we simultaneously performed CT arthrogram with supine and prone position and evaluated which is more reliable and accurate. Scapular inclination was ahout 25 degrees in the supine position and ahout 60 degrees in the prone position. Normal glenoid labrum varied in size and shape, so its lack of a classic appearance might not indicate a labral tear. Variations existed in the anterior capsular attachment onto the glenoid. The anterior capsule was used to attach far away from the glenoid rim. but it might not indicate an anterior instability. With the prone position, the labral lesion took more excellent visualization due to the contrast coating on the glenoid labrum in both anterior and posterior. Anterior capsular expansion was seen well with prone position in anterior instability. In summary, for proper interpretation of the instability, awareness of the morphologic variations of the labrum and capsuloligamentous structure is important. CT double contrast arthrography with prone position has made a significant contribution to improving and easing the documentation of variety of anatomic abnormalities. Now we recommend the prone position in CT arthrogram for imaging glenoid labrum pathology in shoulder instability.