Computed tomographic features of left atrial mural thrombi were compared with those of left atrial myxomas using a third generation system. In seven of 23 patients with mitral valvular disease, computed tomography (CT) demonstrated a total of 12 left atrial mural thrombi. There was calcification in about one-third of the thrombi and marked calcification of the mitral valve in all the seven cases. Mean CT value of the 12 thrombi ranged from +17.5 to +89.8 Hounsfield units (HU) (average +44.5 HU), and in only one it was below +20 HU. All thrombi appeared to be heterogeneous and irregular in shape such as elliptic, triangular or flat on cross-sectional tomogram. In addition, reconstructed coronal and sagittal imagings from serial CT images were very useful for localizing the calcification in the thrombi and determining the site of the attachment of the thrombi to the atrial wall. On the other hand, calcification of the tumor or mitral valve was not detected in two patients with left atrial myxomas by CT. The mean CT value of myxoma was averaged +30.2 HU, which was lower than that of thrombi. Both myxomas were ovoid in shape and appeared to be homogeneous in CT images. ECG gated CT revealed pendular movement of the tumor which prolapsed through the mitral orifice into the left ventricular inflow tract during diastole and backed into the left atrium during systole, although it was slightly difficult to detect the site of the attachment of the tumor. In conclusion, we should evaluate appearances, calcification of the mitral valve, mean CT value, shape and movement in the computed tomographic diagnosis of left atrial masses. In consideration of these characteristic findings, differentiation between mural thrombi and myxomas could be made.