Forty-five consecutive patients with 49 fractures of the first rib caused by blunt trauma underwent arteriography. The fractures were classified as posterior, lateral, or anterior and as nondisplaced, minimally displaced, or significantly displaced. Seven patients (14%) had serious vascular injuries: five had posterior injuries, and one had a lateral fracture, and one had an anterior fracture. All seven had significantly displaced fractures. We postulate that vascular injury can result from a violent lever mechanism in which the posterior portion of the rib is displaced downward and the anterior portion is forced upward, pinching the contents of the thoracic outlet against the clavicle. Four patients with subclavian artery injury had clinical evidence of arterial insufficiency. Branchial plexus injury occurred only in association with vascular injury. These data suggest that only displaced first rib fractures, and in particular posterior displaced fractures, result in vascular injury. Arteriography is indicated if there is an absent pulse, a brachial plexus injury, or a displaced first rib fracture. Using these criteria all vascular injuries would be detected. The majority (78%) of the patients with first rib fractures could be managed without arteriography.