Thrombus in a natural left ventricle during left ventricular assist: another thromboembolic risk factor. Systemic thromboembolism is one of the serious complications during use of a left ventricular assist system (LVAS). The authors' original LVAS has excellent antithrombogenicity to reduce this risk, but thrombus formation in a natural left ventricle (LV) is another risk factor for systemic embolization. We used our LVAS in 22 patients. Of these, LV wall motion was studied in 15, and a smoke-like echo disclosed thrombus in the LV in eight. When LVAS sufficiently supported the systemic circulation, LV wall motion was irregular upon closure of the aortic valve. Upon recovery of LV function and decrease of LVAS flow, the smoke-like echo diminished and LV thrombus decreased in one, but LV thrombus remained unchanged or increased in seven. Systemic administration of antithrombotic agents had an unclear effect. Seven of eight patients with LV thrombus on echo died during or after LVAS. In these seven, a high incidence of LV thrombus (71%), and systemic embolism to the brain (29%) or kidney (86%), were revealed at necropsy. We subsequently used local heparinization to prevent LV thrombus formation. One patient with a smoke-like echo had no evidence of LV thrombus or systemic embolism. During LV assist, careful management of the LVAS, and intra-LV heparinization should be considered to reduce the risk of thrombus formation in the natural LV. 