We report the time-course of gastric mucosal pH (pHi) as assessed by tonometry in a child after surgical repair of a common atrioventricular septal defect. Gastric intra-mucosal acidosis with a pHi below 7.30 persisted for 48 hours after the intervention despite restoration of global haemodynamics. On the third post-operative day a sudden fall in pHi to 7.09 was the first indicator of septicaemia. Hence, pHi tonometry may assume a similar importance in infants after cardiac surgery as in adults.