To assess the use of antithrombotic therapy among atrial fibrillation (AF) patients in a Brazilian University Heart Hospital (InCor). In a cross-sectional study we analyzed the charts of all patients treated at InCor in five separate days of 2002 (Phase 1). To assess the impact of admission to a cardiology hospital, a follow-up of the AF patients selected in Phase 1 was carried out after 1 year (Phase 2). The prevalence of AF in the 3,764 assessed charts was 8.0% (301 patients). In Phase 1, antiplatelets were prescribed to 21.2% and anticoagulant therapy (ACT) to 46.5% of AF patients; in Phase 2, to 19.9 and 57.8%, respectively. Thus, 32.2% (Phase 1) and 22.2% (Phase 2) of AF patients were not receiving any antithrombotic drug. Among AF patients with previous ischemic stroke (17.6%), only 49% (Phase 1) and 60.4% (Phase 2) were receiving ACT. As many as 34 and 22.6%, respectively, were not receiving any antithrombotic drug. After follow-up, a new acute embolic event was documented in 5.6% of patients, 17% died. Anticoagulation is underused in AF patients and neither the fact of being treated by cardiologists in a University Hospital, nor the learning time-window of 1 year seemed to improve the antithrombotic care significantly.