The mechanism of injury to the brachial plexus which follows certain operative positions was studied in 15 cadavers by dissection and in patients by radial artery pulse tracings. In particular the "hands up" and common supine positions were studied. Stretch was shown to be the most likely mechanism leading to injury in these positions. Elevation of the elbows 6 inches from the table prevented stretch in the "hands up" position. Limitation of abduction to 90 degrees and prevention of posterior displacement of the upper arm should prevent injury in the supine position. A mechanism of injury following other operative positions was postulated.