The aim of this study was to investigate the day-to-day reproducibility of 24-hr esophageal pH monitoring. The procedure was performed continuously for 48 hr in order to enable future studies on dietary challenges, using consecutive 2 x 24 hr pH recording. Furthermore, one objective was to relate the degree of reproducibility to endoscopic evidence of mucosal injury. Upper endoscopy and 2 x 24-hr consecutive pH monitoring were performed in 30 infants and children referred for gastroesophageal reflux disease. The monitoring was performed without dietary or activity restrictions in order to assess reflux parameters in a near-normal physiologic setting. The NASPGHAN criteria for pathological reflux index (RI, % fraction of time with pH < 4.0) were employed. Based upon the NASPGHAN criteria for the RI, 9/30 subjects (30%) had discordant (normal vs pathologic) results at the two recording days, yielding an overall reproducibility of 70%. The limits of agreement for RI at day 2 were 0.2-3.3 times the initially obtained value at day 1. No difference was found in terms of reproducibility between groups with and without esophageal mucosal changes (erythema and esophagitis). No significant difference was noted in the association between pH monitoring and macroscopic esophageal mucosal changes between the two recordings days. In conclusion, a considerable intraindividual variability in reflux parameters was observed between the measurements from day 1 to day 2. This physiologic variability should be taken into consideration when evaluating gastroesophageal reflux disease in infants and children by means of pH monitoring. The day-to-day variability limits the use of simultaneous pH monitoring and dietary challenges as a procedure to identify a possible causative relation between GERD and dietary allergy/intolerance.