The reproducibility of 24 hour oesophageal pH monitoring was studied in 30 infants and children including the proportions of the investigation time with a pH less than 4 (reflux index), the number of episodes with a pH less than 4 or lasting longer than five minutes, and the duration of the longest episodes with a pH less than 4. Twenty four hour pH monitoring was performed using identical equipment on two consecutive days under similar conditions. Pearson correlation coefficients range from 0.88 to 0.98. The results obtained on two consecutive days were similar. The reflux index and the number of reflux episodes greater than five minutes were the most reproducible parameters. Despite the many patient related factors influencing pH data, the reproducibility of 24 hour pH monitoring data are satisfactory for routine clinical application.