Background: Cold hemagglutinin disease (CHAD) is a rare autoimmune disease, in which patients manifest symptoms when the body temperature decreases. It causes critical problems with blood clotting and hemolysis during hypothermia in cardiac surgery. Although various methods are recommended, the CHAD discovered incidentally during cardiac surgery is still a clinical challenge. Case presentation: A 76-year-old male visited our hospital for chest pain. Angiography revealed unstable angina, left-main and three-vessel disease. We performed coronary artery bypass graft (CABG) with cardiopulmonary bypass after heparin injection. Shortly after aorta cross-clamping (ACC) and infusion of cold blood cardioplegia, we found massive blood clots in the cardioplegia line. Upon suspicion of CHAD, we raised the temperature and infused warm blood cardioplegia in a retrograde manner. After performing cardiac arrest, we opened the coronary artery and found blood clots in the coronary artery. We eliminated the clots and washed with warm crystalloid cardioplegia simultaneously in an antegrade and retrograde manner. During the ACC, warm cardioplegia was infused every 15 min, via retrograde and antegrade techniques simultaneously. After distal anastomosis of the saphenous venous graft (SVG) to the coronary artery, we performed a direct SVG warm cardioplegia infusion. Finally, before the proximal SVG anastomosis to the aorta, we used warm cardioplegia to eliminate the remaining microemboli. The cold reactive protein test showed a positive result. The patient was discharged without any complications. Conclusion: In this rare case, we incidentally discovered CHAD associated with massive blood clots in the cardioplegia line and the coronary artery, during CABG. However, we performed CABG without any complications using a reasonable and appropriate cardioplegia infusion technique, including direct SVG warm cardioplegia infusion.