Introduction: Deep hypothermia is used in cardiac surgery to ensure the better protection of cerebral and / or myocardial. The threshold recommended minimum is 18°. It is not devoid of consequences whose care requires a multi factorial. Observation: We report the case of a deep hypothermia major 12°, occurred in a patient with a double-valve replacement with mechanical aortic and mitral associated with a plasty tricuspid in the department of thoracic and cardiovascular surgery-vascular Fann. During the second cardioplegia, there has been a mismatch of the temperature of the sick from 34° to 18° on the console CEC (extra-corporeal) with a body temperature of 12° on the scope. No other changes were noted. We conducted a hemodilution with mannitol 0.25 g/Kg, a gradual warming of the patient of a degree every 5 minutes and hemofiltration from 25°. The déclampage aortic has been made once in normothermia to 36° Celcuis. The recovery of heart rate was initially in ventricular fibrillation refractory to medical treatment and to shocks to the heart internal. He had to go back in the CEC with a new aortic cross-clamping and then a half dose of cardioplegia. The recovery of secondary cardiac rhythm was sinus irregular with a hemodynamic stable in the low dose of drug. The postoperative course was marked by a tendency to bleeding contained by the transfusion of blood products. The evolution was favourable with output of resuscitation at J3 and exéat to J5. Conclusion: Hypothermia is a major rare. It is described in accidents and natural disasters. In a medical setting, it is most often of moderate hypothermia. In spite of all the means of monitoring, this major incident occurred, and in our case, without sequelae.