A 48-year-old man presented with pain and sensory impairment radiating from the neck to the thumb and forefinger of the right hand when lifting weights and turning or tilting the head. The symptoms were due to an anomalous accessory part of the trapezius muscle crossing the upper part of the brachial plexus. Excision of the anomalous muscle and release of the clavicular part of the sternocleidomastoid muscle abolished the complaints.