Unilateral phrenic nerve paralysis (PNP) folowed 32 (1.7 percent) of 1,891 consecutive cardiac surgical procedures during an 8 year peroid. Diagnosis was based on radiographic criteria with comparison of preoperative and postoperative chest radiographs and was confirmed in all 21 evaluated by fluoroscopy. Six had persistent radiographic abnormality more than 12 months postoperatively. PNP occurred most frequently in association with Blalock-Taussig shunts. These operations represented 22 percent of this series, and PNP complicated 7 percent of all Blalock-Taussig shunts. PNP was less well tolerated in the 14 infants than in the 18 older children. Eleven infants had serious difficulties during weaning from mechanical ventilatory support. Five infants required tracheostomy, one underwent diaphragmatic plication, and three died. Infants had a mean duration of mechanical ventilation of 24 days and required prolonged intensive care and long-term hospitalization. In comparison, older children had a more benign postoperative course. Diaphragmatic plication should be considered in infants with paradoxical motion of the hemidiaphragm who remain dependent on mechanical ventilatory support for more than 2 weeks postoperatively.