The Bankart lesion is an essential finding of traumatic anterior shoulder instability. The purpose of this study was to clarify the reliability of diagnosis by ultrasonography (US) of the Bankart lesion when using an axillary approach. Six cadaveric shoulders were examined by US from the axilla. The shoulders were then dissected along the plane of the ultrasonic beam. The labrum at the 4-o'clock position of all 6 cadavers was present along the ultrasonic plane; there were no obstacles from the skin to the glenoid rim, and all cases had a normal labrum at the 4-o'clock position. US images of the normal labrum showed that the labrum was continuously connected to the glenoid rim with no borderline (low-echo line) between them. Preoperative US from the axilla was performed in 75 patients who underwent arthroscopic surgery. Anteroinferior labra were classified preoperatively from the US images as normal, detached (low-echo line between glenoid rim and labrum), or displaced (labrum displaced from glenoid rim), with a diagnosis of a Bankart lesion being made for the latter two types. US findings were correlated with arthroscopic observations. This revealed that for the detection of Bankart lesions, US had a sensitivity of 88.6%, a specificity of 77.4%, an accuracy of 84.0%, a positive predictive value of 84.8%, and a negative predictive value of 82.8%. These results demonstrate that US with an axillary approach is useful in the diagnosis of Bankart lesions of the shoulder.