The relationship between the left atrial appendage (LAA) function, as assessed by transesophageal echocardiography, and the incidence of left atrial thrombus was evaluated in 62 patients with nonvalvular chronic atrial fibrillation (AF; n=50) and atrial flutter (AFL; n=12). It was hypothesized that in both AF and AFL not only the LAA flow velocity (LAAFV), but also the frequency of the LAA movement (the LAA flow time, LAAFT) is a major contributing factor to thrombus formation. LAAFT was defined as the average duration of LAA flow with emptying and filling waves. The patients with AF were divided into 2 groups: lone AF (n=14) and non-lone AF (n=36). LAA thrombus was found in 6 patients with none-lone AF. LAAFV was lower and LAAFT was shorter in patients with thrombus as compared with patients without thrombus (12.0±2.2 cm/s vs 24.1±10.6 cm/s, 68.7±1.5 ms vs 72.9±3.3 ms, p<0.01, respectively). Patients with AFL had higher LAAFV and longer LAAFT than those with chronic AF. The present data suggest that, in addition to LAAFV, LAAFT characterized LAA function and might serve as a predictor of thrombus formation in chronic AF. With respect to LAA function, patients with lone AF or AFL are at low risk for thrombus formation. (Circ J 2003; 67: 68 - 72)