The causes of brachial plexus palsy in neonates should be classified according to their most salient associated feature. The causes of brachial plexus palsy are obstetrical brachial plexus palsy, familial congenital brachial plexus palsy, maternal uterine malformation, congenital varicella syndrome, osteomyelitis involving the proximal head of the humerus or cervical vertebral bodies, exostosis of the first rib, tumors and hemangioma in the region of the brachial plexus, and intrauterine maladaptation. Kaiser Wilhelm syndrome, neonatal brachial plexus palsy due to placental insufficiency, is probably not a cause of brachial plexus palsy. Obstetrical brachial plexus palsy, the most common alleged cause of neonatal brachial plexus palsy, occurs when the forces generated during labor stretch the brachial plexus beyond its resistance. The probability of obstetrical brachial plexus palsy is directly proportional to the magnitude, acceleration, and cosine of the angle formed by the direction of the vector of the stretching force and the axis of the most vulnerable brachial plexus bundle, and inversely proportional to the resistance of the must vulnerable brachial plexus bundle and of the shoulder girdle muscles, joints, and bones. Since in most nonsurgical cases neither the contribution of each of these factors to the production of the obstetrical brachial plexus palsy nor the proportion of traction and propulsion contributing to the stretch force is known, we concur with prior reports that the term of obstetrical brachial plexus palsy should be substituted by the more inclusive term of birth-related brachial plexus palsy.