During an 8 1/2 year period, 28 patients with a delayed diagnosis of an arterial injury in an extremity or the neck were treated. The median delay between injury and diagnosis was 10 days. The tibio-peroneal arteries were the most commonly injured vessels. After extensive analysis of the records and arteriograms of the involved patients, the following conclusions were drawn: Arteriograms are mandatory for penetrating wounds proximal to major arteries of the extremities because of the 5 to 15 percent incidence of occult injuries; the timing of arteriography in the distal leg is critical if subtle injuries to the tibial and peroneal vessels are to be detected; when experienced radiologists are not available, interpretation of exclusion arteriograms is best performed by experienced trauma surgeons; false aneurysms, arteriovenous fistulas, or a combination of both continue to be the most common manifestations of missed arterial injuries; failure to find an injury previously diagnosed by a preoperative arteriogram mandates an intraoperative arteriogram and, on occasion, an arteriotomy; and, late arterial repairs usually require segmental resection with an end-to-end anastomosis or insertion of a graft.