In mitral valve disease, it is important to know whether thrombi are present in the left atrium when deciding upon a course of treatment. The left atrial thrombus usually locates in the left atrial appendage. In most cases of mitral valve disease, the left atrial appendage is clearly demonstrated by radionuclide angiography using 99mTc-labeled red blood cells and it can be speculated that the cases in which left atrial appendage are not demonstrated by RNA have left atrial thrombi. On the basis of this hypothesis, the diagnostic accuracy of radionuclide angiography to detect left atrial thrombi was evaluated retrospectively in 60 patients with mitral valve disease who had undergone surgery. The sensitivity of first-pass and equilibrium radionuclide angiography to detect left atrial thrombi was 83% and 67%, the specificity 79% and 54%, and the accuracy 80% and 57%, respectively. Although there were two false-negative cases in which the left atrial thrombi did not locate in the appendage and 10 false-positive cases in which left atrial appendages were not dilated, the negative predictive value was so high that a clearly demonstrated left atrial appendage can be translated into the absence of left atrial thrombi.