BACKGROUND: This study hypothesizes that children with glenohumeral dysplasia secondary to birth brachial plexopathy treated with tendon transfer surgery at a younger age will show greater radiographic improvement than those treated at an older age. METHODS: Twenty-six children treated with latissimus dorsi and teres major tendon transfer had preoperative and 1 year postoperative computed tomography (CT) scans at an average follow-up of 17 months (10 to 46 mo). Average age at surgery was 44 months (10 to 134 mo). CT scans were measured for glenoid version and percent humeral head anterior to the midscapular line (PHHA). Shoulders were classified as dislocated (PHHA 0%), subluxed (PHHA 1% to 35%), or normal positioning (>35%). Two age groups were compared: 24 months or less at the time of surgery (n=11); and greater than 2 years (n=15). Concomitant surgical techniques were also evaluated. RESULTS: For the 11 children treated at 24 months of age or less, PHHA averaged 13% preoperatively (range: 0% to 54%) and postoperatively 42% (range: 25% to 51%); glenoid version averaged -25 degrees preoperatively (range: -36 to -11 degrees) and postoperatively -14 degrees (range: -31 to -2 degrees); and preoperatively 5 shoulders were dislocated, 5 subluxed, and 1 normal; whereas postoperatively 0 shoulders were dislocated, 2 subluxed, and 9 normally positioned. For the 15 children treated at greater than 2 years of age, PHHA averaged 30% preoperatively (range: 0% to 53%) and postoperatively 33% (range: 0% to 57%); glenoid version averaged -17 degrees preoperatively (range: -27 to -4 degrees) and postoperatively -16 degrees (range: -31 to -2 degrees); and preoperatively 2 shoulders were dislocated, 6 subluxed, and 7 normal; whereas postoperatively 2 shoulders were dislocated, 5 subluxed, and 8 normally positioned. CONCLUSIONS: Eleven children treated at 24 months of age or less had significantly greater improvement in the CT scan radiographic measurements of glenohumeral dysplasia, than the 15 children treated at greater than 2 years of age. The effect of age had greater correlation with improvement than open reduction. For children undergoing tendon transfer for lack of active external rotation due to brachial plexus birth injury, improvement in glenohumeral dysplasia can be achieved if the surgery is performed before 2 years of age. LEVEL OF EVIDENCE: Level IV, case controlled series.