Phrenic nerve injury has been reported with the use of iced slush for topical cardiac hypothermia. To study this problem in both valve and coronary procedures, we tried to detect phrenic nerve injury in five groups of patients undergoing cardiac operations in which different techniques of topical hypothermia were used. The results indicate a 24% incidence of left phrenic nerve paresis in patients undergoing coronary bypass with iced slush used for topical hypothermia, 12.5% in patients in whom the cardiac cooling jacket was used in association with cold saline, and 22.9% in patients in whom both the cardiac cooling jacket and iced slush were used in the pericardial sac. There was no phrenic nerve injury when saline alone was used. Phrenic paresis is transient and of no clinical significance except when bilateral. Avoidance of contact of either the cooling jacket or iced slush with the phrenic nerve could avoid this complication.