The blood flow velocity patterns within the left atrial appendage were studied by transesophageal color flow imaging and pulsed Doppler in 84 patients. At the time of the study, 57 of the patients were in sinus rhythm, 25 were in atrial fibrillation, and two were in atrial flutter. The relationships between atrial rhythm, blood flow pattern and the presence/absence of spontaneous echocardiographic contrast or thrombus within the appendage were investigated. Transesophageal echocardiography allowed recording of blood flow velocities in 81 of the 84 patients studied. In 51 of the 55 patients in sinus rhythm the pulsed Doppler study showed a biphasic blood flow pattern, whereas a multiphasic pattern was found in the two patients with atrial flutter and in 14 patients with atrial fibrillation. In four patients with sinus rhythm and 10 patients with atrial fibrillation, no significant blood flow velocity could be detected. Thrombus or spontaneous echocardiographic contrast were found within the left atrial appendage in 20 patients, and in all these patients blood flow was either absent or significantly reduced. Our findings indicate that an absent or low blood flow velocity within the left atrial appendage represents a predisposing factor for thrombosis. Isolated left atrial appendage dysfunction has been documented in four patients during sinus rhythm, which may lead to thrombosis. This observation may offer an explanation for cardioembolic events that occur occasionally in patients without apparent heart disease and sinus rhythm.