MORE accurate diagnosis of venous thrombosis is needed for many reasons, viz, autopsy studies show that fatal pulmonary embolism occurs without overt signs of venous thrombosis. No accurate screening method is available to predict embolism in asymptomatic cases. Prophylactic anticoagulants are recommended for certain high-risk individuals, eg, postoperative or posttraumatic cases, those having congestive heart failure or myocardial infarction—but in some of these, relative or absolute contraindication to anticoagulants exists. In such circumstances a measurement of the risk of thrombosis would be useful. A small but definite risk of venous thrombosis and pulmonary embolism is involved in the use of anovulatory hormones. Changes in certain blood tests which do occur during the use of these drugs may permit the risk in each individual to be defined. New drugs which act on blood platelets are being proposed for the treatment of thrombosis. Changes in blood tests characteristic of thrombosis have been