The study aimed to evaluate the degree of gleno-humeral joint deformation in children with persistent obstetric brachial plexus palsy and its effect on limb function. Computer tomography was performed in 24 children in the mean age of 6.1 years. There were eight boys and 16 girls. Gleno-scapular angle, congruency of gleno-humeral joint and joint deformity according to Waters at all. criteria were measured. The mean functional score according to the Mallet classification system was 12.3 points. The joint was stabile in nine, subluxed in seven and dislocated in nine cases. Gleno-scapular angle in affected joints was 23.3° and in non-affected 4.5°. The glenoid was statistically more retroverted in older children. With more severe posterior incongruence there was statistically greater limitation of passive external rotation, active internal rotation and a poorer functional result according to Mallet. Abnormalities were found also in the humeral head, being deformed and smaller compared to the non-affected side in all cases. Glenoid retroversion, posterior subluxation/dislocation of humeral head and smaller humeral head size are the abnormalities, most often identified in CT examinations. Shoulder function and in particular, passive, external rotation are closely associated with the degree of deformity of the glenoid, as well as with the extent of posterior humeral head dislocation.