In retrograde cardioplegia (RCP), some difficulty is occasionally encountered when inserting a catheter into the coronary sinus (CS). Although the usefulness of transesophageal echocardiography (TEE) for guiding the cannulation procedures has been previously reported by other authors, we have obtained additional findings by TEE monitoring of eleven patients during placement of the CS catheter. The diameter of the CS ranged from 5.5 to 10.7 mm, indicating that it was large enough for the CS catheter to be inserted and that the resistance at insertion was not due to narrow CS. The precise time for inserting the catheter, for which myocardial protection is delayed, ranged from 8 to 376 seconds, with an average of 98 seconds. Dislodgement of the catheter was found in two cases. In case of difficult cannulation, the catheter tip was found to be pushing the right atrial wall adjacent to the CS orifice or alternatively it entered the middle cardiac vein which had a common atrial orifice with the CS in this particular case. We found that the knowing the following technical problems helps appropriate monitoring: the catheter tip becomes unclear when it is not perpendicular to the ultrasonic beam, when surgeon's fingers are placed behind the heart, or when the blood is entirely exsanguinated. Finally we present the possibility of employing images of overflow out of CS during RCP infusion, detected by TEE, as an index of efficient perfusion at the interventricular septum.