The few studies on prognosis of obstetric lesions of the brachial plexus that are not hampered by selection bias or a short follow-up suggest that functional impairment persists in 20-25% of cases, more than commonly thought. Electromyography (EMG), potentially useful for prognosis, is often considered of little value. Denervation in the first week of life has been interpreted as evidence of an antenatal lesion, but is the logical result of the short axonal length affected. EMG performed at close to the time of possible intervention (3 months) usually shows a discrepancy: motor unit potentials are seen in clinically paralyzed muscles. This can be explained in five ways: an overly pessimistic clinical examination; overestimation of EMG recruitment due to small muscle fibers; persistent fetal innervation; developmental apraxia; or misdirection, in which axons reach inappropriate muscles. Further research into the pathophysiology of obstetric lesions of the brachial plexus is needed to improve prognostication. Copyright 2001 John Wiley & Sons, Inc.