Although surgical treatment of brachial plexus birth palsy has yielded encouraging results, persistent inability to abduct and elevate the shoulder is common even in children with excellent return of arm and hand function. The reason for deltoid weakness in the afflicted children is not completely understood and may be multifactorial. Clinical observations, including a pattern of position-dependent weakness, suggest that primary nerve damage may not be the sole cause. The authors performed a retrospective chart study to investigate the outcome of surgical treatment to augment shoulder function in a series of 10 children (ages 9 months to 8 years) with inadequate external rotation of the shoulder and inability to actively raise the arm beyond 90 degrees from a birth brachial plexus injury. At follow-up 6 months after surgery, increased shoulder range of motion was noticed in all, with significantly increased abduction/elevation in 8 of the 10 children. Analysis of data, including pre- and postoperative functional testing and intraoperative electrophysiologic monitoring, led to the conclusion that secondary compression of the axillary nerve in the quadrangular space is a separate and common reason for impairment in children with brachial plexus birth palsy and persistent weakness of the deltoid muscle and may provide an important reason for early intervention.