OBJECTIVE: In Belgium oral anticoagulation therapy is mainly supervised by general practitioners (GPs).This study aims to evaluate the quality of management of oral anticoagulation by Belgian GPs and to verify the relation between time in range and a set of potentially influencing co-variables. METHODS: In a retrospective cross-sectional study, involving 66 GP-practices, the INR-values obtained over a 6-month period were analysed. All INR-values were determined by a single clinical laboratory and additional medical information was provided by the GPs. Linear mixed models have been used to model the patient-specific percentage INR in target as a function of different co-variables. RESULTS: 737 patients were included in the study. Patients who underwent a surgical intervention with an interruption of the anticoagulation during the study were excluded. Patients were only included after the initial starting-up period. 5890 INR-values were obtained. A total of 92,566 days of therapy was evaluated. 50% of the day values were within 0.5 INR-units from target (and 66% within 0.75 INR-units from target). In a multiple regression model, a significant relation between the percentage of time in range and the target INR (2.5 or 3.5) and the gender of the patient was shown. The incidence rate for major bleeding was 5.5/100 patient years (and 3.5/100 patient years for thrombo-embolic events). CONCLUSION: The quality of management of oral anticoagulation by the GPs in Belgium is suboptimal. It is unknown whether interventions such as guidelines, feedback, point-of-care monitoring and computer-assisted anticoagulation monitoring could improve the results.