Background: The purposes of this study were to document the natural history of brachial plexus birth palsy, in relation to the recovery of biceps function, in the first six months of life; to assess the outcome after microsurgical repair of the brachial plexus in patients who had no recovery of biceps function at six months; and to compare the results of transfer of the latissimus dorsi and teres major tendons with the results of derotation osteotomy of the humerus and to compare the results of the tendon transfers and the osteotomy with the natural history of the disorder.Methods: Sixty-six patients (sixty-seven lesions) who had brachial plexus birth palsy were seen for an initial evaluation when they were less than three months old. The time of recovery of biceps function was recorded for each month of life for six months from the date of birth. The patients were divided into groups according to the month of life during which recovery of biceps strength was noted. A physical examination and an assessment with use of the functional criteria of Mallet were performed each month. Microsurgical repair of the brachial plexus was performed in six infants who had no evidence of biceps function within the first six months of life. Another group of twenty-seven patients were referred for evaluation of chronic neuropathy after they were six months old. A transfer of the latissimus dorsi and teres major tendons to the rotator cuff was performed in nine of these patients and a derotation osteotomy of the humerus was performed in seven because of an internal rotation contracture or functional weakness of the external rotators of the shoulder.Results: Twenty-two infants had recovery of biceps function within the first three months of life and had normal function at the time of the latest evaluation. Infants who had recovery of biceps function during the fourth, fifth, or sixth month of life later had significantly worse function, according to the criteria described by Mallet, than those who had had recovery in the first three months (p < 0.005). The clinical results for the six patients who had had microsurgical repair six months after birth were significantly better (p < 0.04) than those for the fifteen patients who had had recovery of biceps function in the fifth month of life. However, the results for the patients who had had repair of the brachial plexus were not found to be better than those for the eleven patients who had had recovery of biceps function in the fourth month of life. The improvement in function, as assessed with use of the Mallet criteria, after tendon transfer (p < 0.001) and humeral osteotomy (p < 0.0001) was significant.Conclusions: The present study confirms the observation of Gilbert and Tassin that it is rare for infants who have recovery of biceps function after the age of three months to have complete neurological recovery. Microsurgical repair was effective in improving function in the small subgroup of patients who had no evidence of recovery of biceps function within the first six months of life.