Injuries to the posterior labrum are less common and more difficult to diagnose compared to anterior labral pathology. This may be in part due to difficulties in preoperative diagnosis. Posterior labral injuries cause abnormal loading of the rotator cuff with subsequent weakness. Examination using the Modified O’Briens test tightens the posterior capsule and posteriorly translates the humeral head, stressing the labrum resulting in pain and weakness. A retrospective case controlled study of 74 patients diagnosed with a posterior labral tear at arthroscopy showed 55 to have subjective weakness on performing a Modified O’Briens test, a sensitivity of 83% and a positive predictive value of 90%. At present there is no single test diagnostic of posterior labral pathology. They can often be missed on MRI scanning and also at surgery if not specifically looked for. We believe that clinically demonstrated weakness on performing a Modified O’Briens test is sensitive, with a high positive predictive value for posterior labral pathology and can help guide further treatment.