BACKGROUND/AIMS: Ambulatory 24-hr esophageal pH monitoring is considered the gold standard for diagnosing gastroesophageal reflux disease (GERD). The aims of this study were to define normal value of proximal and distal esophageal pH monitoring in healthy Koreans and to determine pH criteria that optimally defines acid reflux in the proximal esophagus. METHODS: Twenty-five healthy subjects underwent 24 hr ambulatory esophageal dual probe pH monitoring. The pH values of the proximal and distal esophagus were measured and delta pH in the proximal esophagus was calculated. delta H was then used to determine the likelihood of a true reflux event. RESULTS: In the proximal esophagus, the mean total fraction time at pH <4 was 0.27% and the 95th percentile was 0.95%. In the distal esophagus, the mean total fraction time at pH <4 was 1.42% and the 95th percentile was 3.63%. Proximal esophageal acid reflux was significantly associated with distal esophageal acid reflux when the proximal esophageal reflux occurred at pH <4 and delta H > or = 2. CONCLUSIONS: In healthy Koreans, the reflux parameters were similar to those of Western populations. Optimal criteria for proximal esophageal acid reflux was delta pH > or = 2 and pH <4.