Instability of the shoulder is a disabling problem which may be difficult to evaluate by routine radiographs. As detachment or absence of the labrum can lead to shoulder instability, arthrotomography of the glenoid labrum is a helpful adjunct in substantiating this diagnosis and in planning the choice of surgical reconstruction. We have found excellent correlation between the arthrotomographic findings and the glenohumeral lesion found at operation. Thirty-eight arthrotomograms of the glenoid labrum were performed in thirty-seven patients who had either shoulder pain or instability, or both. The patients were divided into three groups: those with painful subluxation (Group I), those with voluntary subluxation (Group II), and those with persistent shoulder pain (Group III). Twenty-five of the twenty-eight patients in Group I had an abnormality of the labrum shown on arthrotomography. All of the Group-II and Group-III patients had a normal labrum. There were no false-positive arthrotomograms in the eighteen patients in whom a lesion of the labrum subsequently was found at operation. Three patients with a normal labrum on arthrotomography underwent surgical exploration and no abnormalities were found.