OBJECTIVE To evaluate the relationship between the parameters of 24-hour esophageal pH monitoring and gastroesophageal reflux disease (GERD) among elderly subjects. METHODS Twenty-four-hour esophageal pH monitoring was carried out in 20 elderly subjects without apparent GERD symptoms (controls) and 69 suspected GERD subjects. RESULTS Normal values of the parameters from 20 elderly controls were obtained. Percent of total time, percent of supine time and percent of upright time in which the pH was < 4 (indicating reflux) were less than 3.3%, 1.4%, 5.5%, respectively. The number of reflux episodes and episodes lasting longer than 5 minutes were less than 65 and 2 times respectively. The values obtained in 66 GERD suspected subjects were significantly different from those in normal controls. The differences of reflux parameters between the esophagitis group and non-esophagitis group, such as percent of total time with pH < 4, percent of supine time with pH < 4 and number of reflux lasting longer than 5 minutes were also significant. CONCLUSIONS About 51.6% patients (34/66) with reflux symptoms but without endoscopic evidence of esophagitis were definitely diagnosed as GERD by esophageal pH monitoring. Duration of esophageal acid exposure correlated with the severity of GERD.