Left atrial thrombi were evaluated with computed tomography in 23 patients with mitral valvular diseases. In three of the patients, left atrial thrombi were delineated with computed tomography and were confirmed by cardiac surgery or autopsy. The minimum size of the thrombi detected tomographically was 3.5 gm. There were no false-positive or false-negative results with computed tomography in 13 patients who subsequently underwent cardiac surgery. Computed tomography is essentially noninvasive and appears to be one of the best methods to evaluate left atrial thrombus.