Doppler measurement of left atrial appendage (LAA) blood velocity during transesophageal echocardiography has been proposed as a method of assessing LAA contractile function and thromboembolic risk. Clinical and echocardiographic determinants of five LAA Doppler blood velocity patterns were examined in 40 patients with a history of atrial fibrillation (AF), in 10 control subjects, and in 5 patients aged </=60 years having sinus rhythm and left ventricular hypertrophy. In sinus rhythm, two blood velocity patterns were differentiated by the extent of passive emptying of the LAA, which was related to age and left ventricular early diastolic filling properties. In AF, three blood velocity patterns were differentiated by the relative preservation of LAA mechanical function during fibrillatory activity. LAA contractile function is an important but not the sole determinant of blood flow in the normal and fibrillating human LAA.