The present study analyzed the clinical backgrounds of 9 patients with fresh left ventricular thrombus (LVT) detected by two-dimensional echocardiography during the past 5 years. Patients with acute myocardial infarction were excluded. Left ventricular systolic function was disturbed either diffusely or segmentally in all patients with a mean ejection fraction of 33%. In 7 patients, echocardiography was performed shortly after furosemide therapy for New York Heart Association class IV congestive heart failure; echocardiography was also performed just before treatment in 4 of the 7 patients and LVT was not detected in any of them. Two patients died of underlying disorders within 2 months of detection of the thrombus. However, the LVT disappeared in the other 7 patients without any thromboembolic episodes during the 6 months after starting anticoagulant therapy. As fresh LVT developed shortly after diuretic therapy in patients with severe congestive heart failure associated with left ventricular systolic dysfunction, concomitant anticoagulant therapy is recommended.