Twenty-four normal subjects and 64 symptomatic patients with various degrees of reflux disease (24 with reflux symptoms without esophagitis and 21 with mild and 19 with severe esophagitis) underwent quantitative 24-hr intraesophageal pH monitoring. Various reflux parameters during supine, interprandial, and postprandial periods were examined by binary logistic regression and by CART analysis to determine the sensitivity and specificity to separate the various groups of subjects and patients. The distinction was excellent between asymptomatic controls and patients with severe erosive esophagitis (sensitivity and specificity both 100% by logistic regression and 95% and 88%, respectively, by CART), but discrimination was poor when asymptomatic controls were compared to symptomatic patients without esophagitis (71% and 79% by logistic regression and 75% and 92% by CART), which is the most important indication for pH recording in clinical practice. A 3-hr postprandial pH recording was inadequate to distinguish the various groups. The acidity of the reflux episodes during the night appeared to be a crucial factor in the development of severe erosive esophagitis. The duration of esophageal acid exposure was another important factor in the development of reflux lesions.