Four patients with large left atrial masses documented angiographically or pathologically, or both, were studied with M mode echocardiography (four patients) and two dimensional echocardiography (three patients) within 2 to 5 days of angiographic or pathologic diagnosis. The left atrium appeared clear of echos in two patients subsequently documented to have a left atrial thrombus weighing 35 and 100 g, respectively, and located within the body of the left atrium. Definitely abnormal echoes were visualized in a third patient only in the inferior aspect of the left atrium immediately beneath the posterior root of the aorta. Subsequently, a 70 g left atrial myxoma filling almost the entire left atrium was found. In the fourth patient, who had a 125 g left atrial myxoma, the two dimensional four chamber apical view demonstrated tumor filling almost the entire left atrium. Long axis cross-sectional and M mode echocardiograms less clearly demonstrated the extent of the mass. Even large left atrial tumors located within the body of the left atrium may not be apparent or may be underestimated in size by currently available ultrasonic techniques. The relatively homogenous nature of certain masses may be, in part, responsible for the inability to visualize some of them adequately with echocardiography.