SummaryThe authors have studied the prophylactic effect of subcutaneous heparin in weak doses in preventing postoperative deep-vein thrombosis (D.V.T.) in 144 patients. Heparin was administered 2 hours before the operation and every 8 hours during 6 days thereafter, each time with a dose of 5000 U subcutaneously.50 patients were randomised into two groups of 27 control patients and 23 patients treated with heparin. 94 other patients were investigated in a double-blind trial: 47 operated patients received placebo injections and 47 others heparin.D.V.T. was clinically recognized as well as by the labelled fibrinogen technique.In the control group of 74 operated patients, a D.V.T. was clinically recognized in 7 patients, and by the isotopic test in 37 patients. In the treated group of 70 patients, no D.V.T. was clinically recognized but the labelled fibrinogen test recognized a D.V.T. in 11 patients.The authors observed no hemorrhagic complications which could be attributed to heparin. Low-dose of subcutaneous heparin is efficient and safe in preventing postoperative D.V.T.