Twenty-four-hour pH monitoring of the esophagus is frequently performed to assess gastroesophageal reflux. We performed a prospective study to determine if results obtained from stationary and ambulatory pH recording systems are comparable. Two groups of patients were studied. Group I consisted of 12 patients monitored simultaneously by both a stationary and an ambulatory pH recording system, each system having a separate pH and reference electrode. In group II, in order to eliminate electrode variability, 10 patients were monitored simultaneously with both systems and a common single pH and reference electrode. In group I, significant correlations were found in six reflux parameters measured and in the 24-hr composite score (r greater than or equal to 0.8722). However, in three of the 12 patients, marked discrepancies were noted in the composite score calculated by the stationary and ambulatory recording systems. Small variations in the pH level recorded by different pH electrodes may have accounted for the discrepancies. In group II, where electrode variability was eliminated, a better correlation was noted between all parameters measured (r greater than or equal to 0.991), and no discrepancies were noted between calculated composite scores. We concluded that the stationary and ambulatory recording systems tested are comparable in measuring 24-hr esophageal pH.