BACKGROUND: Air trapped in left ventricle(LV) after cardiopulmonary bypass(CPB) is a major source of air embolism. We tried to measure the length of time(T) to remove the air bubbles from release of aortic cross clamp(ACC) and to find the factors affecting the length of time. METHODS: With Institutional Review Board(IRB) approval, 125 patients undergoing valvular replacement and repair of atrial septal defect(ASD) were included in this prospective study. After induction of anesthesia, a 5-MHz phased-array transesophageal echocardiographic(TEE) probe was inserted into the esophagus and then connected to the TEE system. TEE was continuously monitored from the time of release ACC to the end of operation. And the length of time from release of ACC to disappearance of the air bubbles in LV was recorded. RESULTS: The mean T was 27.5+/-12.0 minutes and was statistically longer in patients undergoing mitral valve replacement than in patients with ASD. There was significant difference in T between surgeon 1 and surgen 2. In patients with atrial fibrillation(A-fib) the air bubbles were removed more slowly than in patients with normal sinus rhythm preoperatively and there was negative correlation between preoperative ejection fraction(EF) and the length of time to remove air bubbles(r= 0.23). CONCLUSIONS: Careful management to remove the air bubbles from the LV after release of ACC is required in patients with low EF or A-fib preoperatively and surgeon's attention is required because they are the important factor affecting the length of time to remove the air bubble.