Continuous long-term esophageal pH monitoring has become the preferred test to quantify acid gastroesophageal reflux. Because reflux to a limited extent is physiologic, the determination of optimal thresholds to separate normal from abnormal reflux is mandatory. Esophageal pH was measured during 24 hours in 509 healthy thriving infants, aged 3 days to 1 year, using a glass microelectrode with an external reference electrode connected to a portable recorder. Percentiles of the four parameters studied (reflux index or percent of the investigation time with a pH less than 4, number of episodes with a pH less than 4 during 24 hours, number of episodes lasting greater than 5 minutes, the duration of the longest episode (in minutes) are presented. A percentile curve of the reflux index regarding the age distribution shows that the normal range for the reflux index during the first 12 months of life is about 10% (95 percentile), decreasing from 13% at birth to 8% at 12 months. Application of an age-related percentile curve offers a close-to-reality possibility of data interpretation and illustrates that there is inevitably an overlap of data between normal and abnormal populations, because reflux is a phenomenon occurring to some extent in every human being.