Topical cardiac hypothermia has been shown to be a safe and effective means of providing protection for the ischemic myocardium during aortic cross-clamping. We report herein two cases of postoperative bilateral diaphragmatic paralysis which we believe resulted from hypothermic injury to the phrenic nerves. After open-heart surgery, both patients experienced prolonged weaning from assisted ventilation and severe orthopnea. Return of normal diaphragmatic and phrenic nerve function was demonstrated in one patient ten months after surgery. Failure to correctly interpret the respiratory failure and orthopnea led to confusion and erroneous types of therapy. Awareness of this complication should lead to improved care and postoperative management of patients.