Objective of the study. To evaluate the quality of prophylaxis of relapses of venous thromboembolism in the patients presenting with deep vein thrombosis (DVT). Material and methods. This study was designed to analyse the results of the measurement of the international normalized ratio (INR) in 40 patients treated with warfarin during 3 months after distal deep vein thrombosis. The effectiveness of monitoring effects of anticoagulation therapy was evaluated by the Rosendaal method with special reference to the frequency of recurrent deep vein thrombosis, pulmonary embolism, and complications of anticoagulation therapy. Results. The mean percentage of time during which a patient spent with therapeutic INR values throughout the entire period of observations was 43%. The average therapeutic level of INR was reached only by the end of the second month since the onset of intake of the indirect anticoagulant. The analysis of the data or the patients of different age groups has demonstrated that the time during which INR values remained within the normal range was 38.5% in the patients aged 45—59 years, 46% in those aged 60—74 years, and 32% in the 75—89 year old  patients. The frequency of relapses of venous thrombosis was estimated at 5%, no cases of pulmonary thromboembolism were documented. The hemorrhagic complications developed in 7.5% of the cases. Conclusion. Less than half of the patients treated with indirect anticoagulants during a long period after distal deep vein thrombosis exhibited well apparent anticoagulation effect. As a result, they frequently develop both thrombotic and hemorrhagic complications.