In 334 patients undergoing the Fontan operation, the right atrial pressure was 16 +/- 36 mm Hg (mean value +/- SD) 3 h postoperatively, and was on average higher in those who died or had a takedown of the operation than in those who did not (P = 0.0001). Twenty-four hours after operation, the right atrial pressure was 18 +/- 5.5 mm Hg, was higher in those who died or had a takedown than in those who did not (P less than 0.0001); and in those who died or had takedown it was 23.5 +/- 1.66 mm Hg and higher than at 3 h postoperatively (18.7 +/- 0.52). The left atrial pressure 3 h postoperatively was 9 +/- 3.6 mm Hg, and on average was higher in the patients who died or had a Fontan takedown than in those who did not. The continuous relation between right atrial pressure and the probability of death or takedown during the first 24 postoperative hours was such as to recommend consideration of takedown whenever the right atrial pressure reaches 22 mm Hg, and when higher, the recommendation is made with greater urgency.