Two hundred twenty-seven patients who underwent open heart surgery with retrograde continuous cold blood cardioplegia (RC-CBCP) were investigated to manifest the incidence of postoperative conduction disturbances (CD) and to determine factors related to the occurrence of CD. The incidence of CD in patients with RC-CBCP was 16.7%, which was lower than that (24.4%) in 41 patients with antegrade continuous cold blood cardioplegia. In addition, the CD resolved by the time of hospital discharge in 65% of the patients. There were no patients with new complete atrioventricular block requiring a pacemaker. The development of postoperative CD was related to topical cooling with slushed ice and lower myocardial temperature of the left ventricle, but not to kinds of diseases, duration of aortic cross-clamp, or the distribution of RC-CBCP evaluated from myocardial temperature at the end of initial infusion of cold cardioplegic solution. Furthermore, terminal warm blood cardioplegia reduced the occurrence of CD. We conclude that the occurrence of CD in patients with RC-CBCP is lower than that in patients with antegrade cardioplegia, and is most related to local hypothermia with slushed ice. This suggests that the most likely mechanism for the development of CD would be ischemic or reperfusion injury to the specialized conduction system with the disturbance of microcirculation because of rouleaux formation in CBCP at very low myocardial temperature.