A prospective study of a consecutive series of 100 patients in whom a phrenic nerve protector was used during cardiac surgery under CPB demonstrated the existence of one case of postoperative left phrenic nerve palsy and three cases of postoperative right phrenic nerve palsy. Comparison with a previous series taken as the control revealed a very significant reduction in the incidence of left phrenic nerve palsy which decreased from 8% to 1%. The incidence of right phrenic nerve palsy was not significantly decreased which corroborates the role of pericardial cooling in the aetiology of these palsies and the absence of protection of the right phrenic nerve by the system used.