Study of the statistical results of numerous multicentre trials has led the author to propose, as preventive therapy for massive postoperative pulmonary embolism, the employ of 5,000 I.U. of calcium heparin two hours before operation, followed by the same dose every 12 hours until the patient is active enough to be discharged. High risk patients (previous history of thrombo-embolism, malignant affections...) should be given 5,000 I.U. of calcium heparin every 8 hours. Associating dihydroergotamine with the calcium heparin doses enables reduction in dosage of the latter, 2,500 I.U. of the heparin combined with 0.5 mg of D.H.E. being sufficient when given every 12 hours in patients undergoing internal surgery, and 5,000 I.U. combined with 0.5 mg of D.H.E. being required only at 12 hours intervals in high-risk patients.