The efficacy of low-molecular-weight heparin as a prophylactic agent was assessed in 150 consecutive patients over the age of 40 undergoing major abdominal surgery. Fifty of these patients received 1250 activated partial thromboplastin time (APTT) units of low-molecular-weight heparin every 12 hours: three developed isotopic deep vein thrombosis, which was confirmed by phlebography in two cases. The other 100 patients received a single injection of 1850 APTT units of low-molecular-weight heparin. Three of them developed isotopic deep vein thrombosis; phlebography failed to confirm the presence of thrombi in each case. None of the 150 patients studied died from fatal or contributory pulmonary emboli. Low-molecular-weight heparin was not associated with any increase in preoperative or postoperative bleeding. The effect of equal amounts of postoperative bleeding. The effect of equal amounts of low-molecular-weight heparin and unfractionated heparin on the coagulation mechanism during surgery was investigated in another 30 patients. The clotting assays and results of in-vivo platelet function tests indicated that both preparations produced similar effect. Intragroup comparisons, however, showed significant differences in the anti-factor Xa activity, lipoprotein lipase release, and plasma prekallikrein concentrations. A single injection of low-molecular-weight heparin daily is a convenient way of preventing deep vein thrombosis in high-risk patients undergoing major abdominal surgery.