Fifty-four patients with nonrheumatic atrial fibrillation (AF) were studied: 16 patients with (group I) and 38 patients without (group II) documented systemic embolism. Transesophageal echocardiography (TEE) was performed to evaluate the presence of left atrial (LA) appendage thrombus and LA spontaneous contrast, LA size, systolic and diastolic peak velocity of the left pulmonary vein, and forward and backward peak velocity of the LA appendage. No difference was observed in the presence of LA thrombus between the 2 groups. The occurrence of LA spontaneous contrast was significantly (p = 0.01) higher in the group with embolism. LA size, measured by atrial length (4.96 +/- 0.84 vs 4.79 +/- 1.38 cm; p = NS) and atrial width (4.50 +/- 0.96 vs 4.31 +/- 1.24 cm; p = NS), was the same for both groups and thus not associated with embolism. There was no difference in systolic peak velocity (0.39 +/- 0.22 vs 0.44 +/- 0.22 m/s; p = NS), and a trend toward a higher diastolic peak velocity (0.50 +/- 0.17 vs 0.42 +/- 0.15 m/s; p = 0.08) was seen in the left pulmonary vein in the group with embolism. Forward (0.25 +/- 0.19 vs 0.39 +/- 0.23 m/s; p < 0.05) and backward (0.23 +/- 0.15 vs 0.33 +/- 0.16 m/s; p < 0.05) peak velocities of the LA appendage were significantly lower in the embolism group. Assessment of LA appendage flow velocity may potentially identify patients with nonrheumatic AF at high risk for systemic embolism.