Thirteen infants with congenital brachial plexus palsy (eight with upper, five with upper and lower) were monitored by magnetic resonance imaging (the first performed between 7 and 41 days of age and the second at 3 months of age), electromyography (the first performed between 27 and 50 days and the second at 3 months), and the muscle scoring system of the Hospital for Sick Children (at 3, 6, and 9 months of age). The findings were evaluated with respect to the clinical status of the patients at 12 months of age. Magnetic resonance imaging, which could be performed readily even in the neonatal period, revealed pseudomeningoceles in two of the five patients with a poor prognosis (in all planes even in the early days after birth) and in two of the eight patients with a good prognosis (more easily visible at 3 months of age). Electromyography implied root avulsion in three of five patients with a poor prognosis. Electromyography can be of great value for patients with a poor prognosis and root avulsion but may underestimate the severity. The muscle scoring system (Hospital for Sick Children) was determined to be the most predictive method for prognosis.