To correlate blood flow velocities with thrombogenesis in the left atrium (LA) and left atrial appendage (LAA), we performed transesophageal two-dimensional echocardiography (Eso 2-D Echo) combined with pulsed Doppler flowmetry, which was developed in our laboratory. Thirty-eight patients were studied; 1) 15 who had sinus rhythm including four with mitral stenosis (MS) and 11 normal subjects, and 2) 23 with atrial fibrillation (Af). The patients with Af were classified as lone Af (nine cases); Af-CI: Af with cerebral infarction having no MS (four cases); and Af-MS: Af with MS (10 cases). In four of 10 patients with Af-MS, mural thrombi were detected in the LA or LAA by Eso 2-D Echo, but they were not visualized by conventional echocardiography. In lone Af and Af-CI, however, abnormal echoes in the LAA were not observed by Eso 2-D Echo. Thus, Eso 2-D Echo proved superior to conventional 2-D echocardiography in detecting mural thrombi in the LA or LAA. In sinus rhythm, a biphasic flow pattern followed the P wave on the ECG. It was observed in the vicinity of the junction between the LA and LAA. In Af a flow with a saw-tooth appearance was observed throughout the cardiac cycle. The peak flow velocity in the LAA was significantly less in Af than in sinus rhythm (p less than 0.05). Especially in Af-MS, the peak flow velocity was markedly decreased (1 +/- 4 cm/sec: mean +/- SD) compared with those of the other Af groups (p less than 0.01), and it was significantly less in Af-CI (11 +/- 5 cm/sec) than in patients with lone Af (21 +/- 9 cm/sec) (p less than 0.05). These results indicate that analysis of blood flow velocities in the LAA by Eso 2-D Echo is of great value in assessing thrombogenesis in the LAA, and the patients with Af, even without MS, who have markedly reduced peak flow velocities in the LAA, should be prophylactically anticoagulated, because stagnation of blood is strongly suspected.