OBJECTIVE To evaluate the features of the posterior labrocapsular periosteal sleeve avulsion (POLPSA) lesion on MR imaging in athletes with posterior shoulder instability. METHODS Six male athletes (age range 19-43 years) with avulsion of the posterior glenoid periosteum were identified on MR imaging. There were four football players, one wrestler, and one competitive weightlifter. The weightlifter had a bilateral condition so that seven shoulders were evaluated. MR imaging was performed with a 1.5 T magnet utilizing conventional and fat-saturated fast spin-echo coronal oblique and sagittal oblique sequences and a 3D-GRE transaxial sequence. Surgical correlation was available in all shoulders. RESULTS All patients presented with pain and a joint effusion. The size of the periosteal sleeve and redundant joint recess was variable. Fibrous proliferation was noted arthroscopically in four shoulders beneath the sleeve. Although the posterior labrum was detached in all studies, only one labrum had a tear while two showed marked degeneration. CONCLUSIONS The POLPSA lesion is an abnormality that can be associated with posterior instability. It differs from a reverse Bankart lesion because the periosteum, although detached, remains intact with the posterior capsule and detached posterior labrum. This lesion may represent an acute form of a Bennett lesion.