This article reports two clinical cases with the opposite outcomes of the standard anticoagulant treatment by low molecular weight heparin. Two similar patients with the high risk of venous thromboembolism (VTE) were admitted to the hospital for the pre-planned surgical treatment of colorectal cancer. During the perioperative period they underwent the combined treatment intended to prevent postoperative VTE. The treatment included the subcutaneous administration of enoxaparin at a dose of 40 mg/day and the use of compression anti-embolism stockings (18—21 mm Hg). In one case this treatment was complicated by the development of the thrombosis of crural veins detected by ultrasound angioscanning on day 7 after surgery. The second patient experienced bleeding from the surgical wound on day 3 after the intervention that required surgical hemostasis. The blood coagulation system state was evaluated in dynamics by the standard coagulation tests (aPTT, INR), measurement of anti-Xa-activity, and the thrombodynamics test. The retrospective analysis has demonstrated that all using assays in the patient suffering hemorrhage registered pronounced hypocoagulation. At the same time, the patient with thrombosis had normal aPTT, INR suggested mild hypocoagulation, the anti-Xa-activity reached the target values, and only the thrombodynamics test revealed the persisting hypercoagulation under conditions of enoxaparin therapy. Thus only the thrombodynamics test allowed detecting the deviations in the hemostasis system state of both patients that lead to the opposite results of identical prophylaxis of postoperative VTE.