The risk of venous thromboembolism after trauma is thought to be high, but the specific risk factors and the incidence of venous thromboembolism in the trauma population are poorly defined. Between October 1, 1987, and March 1, 1988, 719 patients were evaluated; 542 had no risk factors and 177 had at least 1 risk factor. No venous thromboembolism occurred in any of the 542 patients without a risk factor, whereas 12 of 177 patients (7%) with at least 1 risk factor had a venous thromboembolism. Pneumatic compression hose was the most common form of prophylaxis used, but it could not be applied to 35% of limbs because of plaster immobilizers, external fixators, complex wounds, or traction. In the high-risk group, 25 patients (14%) received no prophylaxis because of a physical impediment to application of these hose and a contraindication to anticoagulation. Age greater than 45 years was the only risk factor predictive of venous thromboembolism by logistic regression analysis. Patients with more than one risk factor had a significantly higher incidence of venous thromboembolism than those with only one risk factor. We conclude that a selected subgroup of trauma patients appears to be at risk of venous thromboembolism and should receive prophylaxis. Approximately one in seven high-risk patients cannot receive anticoagulant or mechanical prophylaxis because of their injuries.