An open controlled study was carried out to assess the efficacy and tolerance of a new low molecular weight heparin for the prevention of post-surgical deep vein thrombosis and pulmonary embolism. Forty-five patients undergoing abdominal surgery mainly for neoplasm, gallstones and gastric ulcers were administered 7,500 AXaU of low molecular weight heparin subcutaneously, 2 hours before surgery and once a day for 7 days after. Heparin calcium (15,000 IU subcutaneously per day) was used as a comparison drug in 45 control subjects, matched for age, sex and type of operation. Deep vein thrombosis was identified with clinical parameters, radio-labelled fibrinogen uptake test, echo-doppler and venography; pulmonary embolism with clinical examination, chest X-rays and/or scintigraphy. No episodes of deep vein thrombosis occurred in the low molecular weight heparin-treated patients, whilst there was 1 episode, without pulmonary embolism, in the control group. The consumption of blood and haemoderivatives for transfusions was higher in the heparin calcium group. Only in this group, furthermore, did 5 patients have to suspend antithrombotic treatment due to severe haemorrhages. General tolerance of the two drugs was identical and very good.