Data regarding left atrial (LA) and LA appendage anatomy and function among patients with newly recognized atrial fibrillation (AF) who have not received long-term warfarin are currently unknown. To identify echocardiographic indexes which characterize those at increased risk for thrombus formation, we analyzed transesophageal echocardiographic studies in 100 consecutive patients with newly recognized AF (duration 2.6 +/- 0.3 week) who had not received long-term warfarin. Fourteen percent of patients had LA thrombi. LA thrombi were associated with larger LA appendages, more depressed LA appendage outflow velocities, and a higher prevalence of severe spontaneous LA contrast. Patients with spontaneous contrast had larger LA and LA appendage anatomy and lower LA appendage ejection velocity. Among patients presenting with their first episode of AF, greater LA appendage ejection and filling velocities and smaller LA and LA appendage sizes were seen among those with AF of <2 weeks duration compared with those with AF of >2 weeks. Thus, patients with recent onset AF and LA thrombi or spontaneous echo contrast have more dilated LA and LA appendage anatomy, and more depressed LA appendage systolic function. Data from patients with their first episode of AF suggests that AF is associated with rapid LA remodeling.