BACKGROUND: Inappropriate and subtherapeutic anticoagulants dosages may result in severe thromboembolic and bleeding complications. The use of this treatment requires special attention and strict clinical and laboratory follow-up. OBJECTIVE: To identify factors associated with appropriate control of the oral anticoagulant use, assessing the patients' knowledge and perception of the treatment. METHODS: A cross-sectional study which included 140 patients followed in the oral anticoagulation outpatient clinic from November 2005 to June 2006. A structured questionnaire was drafted and applied to obtain the clinical characteristics of the patients and their knowledge about the treatment, their compliance with the treatment (Morisky s test) and their perception of the treatment. RESULTS: The main indications for the use of oral anticoagulation therapy were atrial fibrillation (61.4%) and a prosthetic heart valve (55%). The duration of anticoagulation ranged from 24 to 72 months, and phenprocoumon (58%) was the most commonly used anticoagulant. As to the perception of the treatment, 95% of the patients mentioned concern about daily use of this medication. Periodic blood tests (21.4%) and the strict intake of oral anticoagulant (12.8%) were considered limiting factors. Adequate knowledge was outstanding in patients with an international normalized ratio (INR) outside the therapeutic range (64%), compared to patients with an INR within the therapeutic range (62%), as well as compliance with treatment in patients with an INR within the therapeutic range (54%), but with no statistical significance. CONCLUSION: The results of this study show a prevalence of patients using oral anticoagulant with an INR within optimal values, although a high percentage of patients do not comply with the treatment. In this population it is clearly seen that they do not fully understand the treatment.