OBJECTIVES: Describe prevalence and characteristics of patients with chronic nonvalvular atrial fibrillation (NVAF) and ≥1 risk factor (RF) for thromboembolism in France in 2004, and document reasons for vitamin K antagonists (VKA) underuse in those patients for whom anticoagulation is indicated. METHODS: Cross sectional study. All patients with arrhythmia or AF presenting to cardiologists (n = 43) from the THALES observatory during a 3-month period were evaluated via questionnaire. Statistical comparisons were by chi2 and ANOVA analyses. Data were extrapolated using the THALES database to give representative national values. RESULTS: 409 patients met the inclusion criteria, ie, confirmed NVAF and ≥1 thromboembolic RF (mean age 76 years; mean number of thromboembolic RF 2.11; 58.7% male). Of these, 37.2% had 1 RF, 28.9% had 2 RF and 34.0% had ≥3 RF. VKA was prescribed to 65.5% of patients. VKA treatment was associated with a higher mean number of RF (2.28) than aspirin (2.20) or no treatment (1.61). Among VKA-treated patients, 18.6% were judged difficult to stabilise/not stabilised by the physician. Main reasons for not prescribing VKA were insufficient risk:benefit (37.6%), patient's refusal due to VKA restrictions (23.4%) and patient's inherent haemorrhagic risk (19.1%). According to anticoagulation exclusion criteria (severe hepatic insufficiency, recent stroke, patient's inherent haemorrhagic risk) most patients not treated with VKA (77.3%) would have been eligible for anticoagulation. Extrapolation of these findings to the French population equated to 426,731 (95% CI 420,099−433,363) patients with chronic NVAF. Of these, 147,124 (34.5%) would not be treated with VKA, even though 113,719 would have been eligible for anticoagulation. CONCLUSIONS: These data suggest that one third of French patients with chronic NVAF and ≥1 thromboembolic RF presenting to cardiologists are currently not treated with VKA. Main drivers for this non-prescription are more related to risk and constraints of VKA treatment than formal contraindications.