Left spinal accessory nerve palsy occurred in a young man when he quickly turned his head to the right while his shoulders were pulled down by heavy hand-held objects. Electrophysiologic studies demonstrated partial axonotmesis of the spinal accessory nerve branches innervating the sternocleidomastoid and upper and middle trapezius and complete axonotmesis of spinal accessory branches to the lower trapezius. There was a separate, although functionally minor, cervical plexus innervation of the lower trapezius.