The presence of a left atrial thrombus is considered to be a relative contraindication to percutaneous transvenous mitral commissurotomy (PTMC) in patients with rheumatic mitral stenosis. However, resolution of left atrial thrombus after anticoagulant therapy with warfarin makes PTMC possible. From July 1989 to June 1991, a total of 70 patients with rheumatic mitral stenosis received PTMC at National Taiwan University Hospital. Of these, four patients underwent PTMC uneventfully after resolution of left atrial thrombi with anticoagulant therapy. The prothrombin time was kept at around 1.5 times that for the normal controls and transesophageal echocardiography (TEE) was used for follow-up. The time for resolution of left atrial thrombi was 1.5, 11, 12, and 2 months. In all four patients with chronic atrial fibrillation, TEE revealed the presence of left atrial thrombi; in only two of these cases was there a suspicion of left atrial thrombi on transthoracic echocardiography. It is concluded that: (1) left atrial thrombi may be resolved after anticoagulant therapy with warfarin, but the time required varies for different patients; and (2) TEE is better than conventional transthoracic echocardiography for detecting a left atrial thrombus and is recommended as the tool of choice for observing the response of a left atrial thrombus to anticoagulant therapy.