The proposed project will consider various methods for prevention of venous thromboembolism in high risk operative and nonoperative patients, including antithrombotic drugs and physical methods such as external pneumatic compression of the limbs. Controlled prospective trials of prophylactic methods will be conducted in patients assigned at random to treatment groups. Diagnostic methods will be sensitive objective techniques including radioactive fibrinogen scanning, impedance plethysmography, and radiographic phlebography. Particular attention will be payed to the role of platelets in venous thromboembolism.