Objective To compare the myocardial protective effect of warm and cold blood cardioplegia during open heart surgery with cardiopulmonary bypass ( CPB) . Methods Thirty patients with congenital heart disease undergoing open heart surgery under CPB were randomly divided into warm and cold blood cardioplegia groups. In both groups the ratio between oxygenated blood and cardioplegic solution was 4 : 1. In warm blood cardioplegia group the heart was perfused with warm blood (35℃) mixed with high-potassium cardioplegic solution as soon as the aorta was cross-clamped until cardiac arrest developed (flat baseline on ECG). Then the heart was perfused with cold blood (4-81) mixed with low-potassium cardioplegic solution 10 ml?kg-1 every 20 min during ischemic period. At the end of CPB warm blood without potassium was again used to perfuse the heart. Venous blood samples were taken 0.5 h before operation, at the end of operation (T1) and 3 h, 1, 3 and 6 days (T2-5) after surgery for determination of plasma concentrations of CK-MB, cTnI, TNF-?, IL-6 and IL-10. The rate of spontaneous recovery of heart beating was compared between the two groups. Results The two groups were comparable with respect to the demographic data and CPB time. The rate of spontaneous recovery of heart beating was significantly higher and the duration of postoperative mechanical ventilation and ICU stay were significantly shorter in the warm blood cardioplegia group than in cold blood group. The plasma levels of CK-MB, cTnl, TNF-?, IL-6 and IL-10 increased significantly after operation as compared to the baseline (T0) in both groups. The plasma concentrations of cTnl, CK-MB, TNF-? and IL-6 were significantly lower after operation while plasma IL-10 level was significantly higher at T3-4 in warm than in cold blood cardioplegia group ( P