Nineteen patients with 20 clinically occult arterial injuries were prospectively followed without surgery in order to define the natural history of these lesions. The patients were predominantly male (74%), with a mean age of 31 years. The mechanism of injury was penetrating in 13 cases and blunt in seven cases. The arterial injuries were located in the lower extremity (45%), upper extremity (35%), neck (15%), and abdominal aorta (5%). Neurologic trauma (55%) and musculoskeletal trauma (40%) were the most common associated injuries. There was no clinical manifestation of vascular injury in any case. All lesions were identified by arteriography, which was performed for high-risk blunt trauma and proximity of penetrating wounds to major vessels. Intimal flap was the most commonly demonstrated form of injury (65%), followed by focal narrowing (30%) and false aneurysm (5%). Followup of 19 injuries (95%) was obtained for periods ranging from 3 days to 19 months (mean, 3.8 months). Arteriographic followup was obtained in 15 cases (79%). One injury (5%), a false aneurysm, enlarged after 10 weeks and was surgically repaired without clinical sequelae. All other lesions either resolved (53%), improved (16%), or remained unchanged (26%). These results suggest that nonoperative observation may be a safe and feasible method of managing clinically occult arterial injuries.