A b s t r a c t Introduction: Deep venous thrombosis (DVT, phlebothrombosis) is a very important clinical problem with its resultant fatal pulmonary embolism (PE) as one of the possible consequences. Factor V Leiden (FV Leiden) is a genetic disorder characterized by a poor anticoagulant response to activated protein C (APC) and an increased risk of venous thromboembolism (VTE). Homozygous carriers of the FV Leiden mutation are estimated to have an 80-fold increased lifetime relative risk of VTE. Most homozygous carriers present with VTE before 40 years of age, but some can live thrombosis-free until the sixth or seventh decade of life or even remain asymptomatic for life. Case-controlled studies of patients with cancer revealed a four-fold increase in thromboembolic occurrence in acute leukaemia, with the risk of thrombosis persisting even after remission of the disease.Case Report: The authors present a case report of the 61-year-old patient with leukaemic transformation of myelodysplastic syndrome (MDS) to acute myeloid leukaemia (AML) and development of DVT of the left leg (LL) as the first clinical manifestation of homozygous FV Leiden carrier. Due to his diagnosis it was not possible to indicate surgical treatment of DVT. After initial treatment with subcutaneous low molecular weight heparin (LMWH) and continuous intravenous application of unfractionated heparin (UFH) the deficiency of antithrombin III (AT III) potentiating the persistence of his hypercoagulable state developed. Treatment with the new oral anticoagulantrivaroxaban, vasoprotective and antithrombotic drugs in the combination with mechanical methods of VTE prophylaxis led to a reduction in edema of left thigh and calf by 3.5 cm and 4 cm, respectively. Son of the patient experienced DVT at the age of 27 years, too.Conclusion: In this report, we describe a case of the patient with DVT during the leukaemic transformation of MDS to AML as a relatively late first clinical manifestation of homozygous FV Leiden mutation. At the same time the article deals with the clinical aspects of discussed thrombophilia in the relatives of the patient, as well as with the etiopathogenesis, pharmacologic treatment options and possible complications of DVT (PE, pulmonary hypertension and post-thrombotic syndrome).