Objective: We performed a randomized clinical trial to evaluate the efficacy of HTK solution萸� effects of postoperative myocardial protection after congenital heart surgery in comparison with blood cardioplegia using electron photomicrographs. Methods:Twenty two patients programmed for elective open heart surgery were randomized into two groups. Group HTK(n =11 ) received HTK cardioplegic solution ;group blood(n = 11) received conventional blood cardioplegic solution during heart surgery. A small biopsy specimen was taken from the right ventricle and was processed for ultrastructural examination after reperfusion. Electron photomicrographs were made by an investigator who was totally blinded to the study design. Semiquantitative electron microscopy was carried out blindly in 4 areas per specimen and in 5 test fields per area by ���random systematic sampling���. Then the morphology of the mitochondrial membrane and crista are scored. The interstitial edema of myocardium also are graded. Results: The semiquantitative score of mitochodrial morphology was 19.65 ?4.75 in group blood and 25.25 ?5.85 in group HTK (P=0.03). 6 patients(54.5%) in blood group and 3 patients(27.3%) in HTK group were grade 3 or more in the interstitial edema of myocardium. Conclusions: : Ultrastructural integrity was preserved even better with HTK solution than with conventional blood cardioplegic solution. These results support the superiority of HTK solution over blood cardioplegia for myocardial protection during congenital heart surgery.