The association between internal rotation contracture secondary to brachial plexus birth palsy and deformity and posterior dislocation of the glenohumeral joint has been known for a long time. The precise nature of these deformities and their pathogenesis, however, remain unclear. Twenty-five children, ranging in age from 1.5 to 13.5 years, had an operation to release an internal rotation contracture secondary to brachial plexus birth palsy; eleven had a latissimus dorsi transfer to augment external rotation power as well. Arthrograms were made intraoperatively in order to clarify the pathological changes that occur in the glenohumeral joint during growth in patients who have this condition.Seven children had a concentric glenohumeral joint (the humeral head was well centered in the glenoid fossa). The remaining eighteen children (72 per cent) had a deformity of the posterior aspect of the glenoid. Five of these children had flattening of the posterior aspect of the glenoid, seven had a biconcave glenoid with the humeral head articulating with the posterior of the two concavities, and six had a so-called pseudoglenoid (the most severe deformity, in which the humeral head articulated with a distinct, retroverted, posterior articular surface).Internal rotation contracture secondary to brachial plexus birth palsy may lead to glenoid deformity that is severely advanced by the time that the child is two years old. In patients who have such a contracture, we recommend early imaging of the shoulder with arthrography or some other modality to allow visualization of the skeletally immature glenohumeral joint.