The results of ambulatory 24-h esophageal pH monitoring in 67 patients with gastroesophageal reflux symptoms (endoscopic esophagitis, n = 44; normal endoscopy, n = 23) were compared with those of 27 normal subjects without reflux symptoms. Patients with reflux symptoms had significantly increased gastroesophageal reflux compared with normal subjects. Acid reflux time was significantly (p less than 0.001) correlated with the severity of endoscopic esophagitis. Linear discriminant analysis was used to differentiate, for each reflux variable, between patients and controls. When the percentage of overall time at pH below 4 was used as a single determinant of gastroesophageal reflux, the sensitivity and specificity were 81% and 85%, respectively, with 4% as upper limit of normal. Pathologic reflux was found in 61% of the patients with negative endoscopy. Long-term ambulatory pH-metry is of clinical value in detecting pathologic reflux in symptomatic patients with negative endoscopy.