Ambulatory 24-h esophageal pH monitoring is increasing in popularity as the means to measure esophageal exposure to gastric juice and document the presence of gastroesophageal reflux disease, particularly before surgical therapy. Normal values for pH exposure were obtained from 50 asymptomatic healthy subjects. Receiver operating characteristic curves constructed from another 25 asymptomatic healthy subjects and 25 selected patients with other markers of increased esophageal acid exposure showed that a composite score and the percent total time pH less than 4 provide the most efficient interpretation of the test with a sensitivity of 96%, a specificity of 100% and an accuracy of 98% for the composite score, and a sensitivity, specificity, and accuracy of 96% for the percent total time pH less than 4. Repeat monitoring of healthy volunteers and symptomatic subjects in the inpatient and outpatient environment showed no significant difference, with the exception that the number of reflux episodes was significantly greater during the outpatient recording in volunteers. This did not affect the clinical accuracy of the test. Esophageal pH probes were well tolerated, but caused belching and coughing during the early part of the monitored period. We conclude that computerized ambulatory 24-h esophageal pH monitoring in the outpatient setting provides accurate and reproducible results.