Background/Aims: Twenty-four-hour ambulatory esophageal pH monitoring is considered the gold standard for diagnosing gastroesophageal reflux disease. The aim of this study was to quantify normal distal esophageal acid parameters in healthy Koreans. Methods: Thirty healthy adults who were not on medication and were free from gastrointestinal symptoms were analyzed. Ambulatory esophageal acid (pH ＜4) exposure parameters were recorded at 5 cm above the lower esophageal sphincter. Results: The 95th percentiles for reflux parameters assessed in the distal esophagus were as follows: percentage of total time with pH ＜4, 5.10%; percentage of upright time with pH ＜4, 7.88%; percentage of supine time with pH ＜4, 4.00%; number of reflux episodes, 62.7; number of reflux episodes with pH ＜4 for ＞5 minutes, 5.3; and the longest single acid-exposure episode, 21.3 minutes. Conclusions: Physiological gastroesophageal reflux occurs frequently in healthy Koreans. These data provide a reference range that could be utilized in studies involving Korean subjects. (Gut and Liver 2008;2:8-13)