Investigation of gastro-oesophageal reflux often includes endoscopy, usually under general anaesthesia, and pH monitoring. In most cases, the pH probe is passed when the child is awake and is poorly tolerated. The effect of general anaesthesia on pH monitoring is unknown. The aim of the study was to determine if placing the probe in the anaesthetised child gives a representative pH study. Twenty children aged 4 months to 13 years underwent oesophago-gastroduodenoscopy under general anaesthesia. A pH electrode was placed under direct vision in the distal oesophagus. pH monitoring was begun after completion of anaesthesia and continued for 18-24 hours. The study was repeated within 14 days without anaesthetic. The reproducibility of values of percent pH < 4, number of reflux episodes/hour, reflux episodes lasting > 5 min, and longest reflux episode was 85%, 90%, 75%, and 75% respectively. These results are comparable with those in adults and children in whom pH studies were performed on consecutive days (without anaesthetic) keeping all variables constant. Therefore pH data collected in a child within 24 hours of endoscopy under general anaesthesia are representative.