To evaluate the diagnostic value of different tests for gastroesophageal reflux disease, a test population was constructed from 45 patients with symptoms of heartburn and regurgitation with or without esophagitis and 45 healthy subjects, who never experienced heartburn, regurgitation, or swallowing discomfort. The test population underwent esophagoscopy, standard acid reflux test, 24-hour pH monitoring, and manometry of the lower esophageal sphincter. Sensitivity, specificity, positive predictive value, negative predictive value, and the accuracy of the tests and test combinations were calculated. Esophagoscopy had a sensitivity of 62%, that is, only 62% of patients with the disease have evidence of mucosal damage on endoscopy. Manometric measurements of the lower esophageal sphincter had a sensitivity of 84%, a specificity of 89%, and an accuracy of 87%. Twenty-four hour esophageal pH monitoring had a sensitivity, specificity, and accuracy of 96%. The results show that 24-hour pH monitoring can detect gastroesophageal reflux disease with an accuracy of 96% by measuring an increase in esophageal acid exposure. Manometry of the lower esophageal sphincter can detect a mechanically deficient sphincter as a cause of the disease with an accuracy of 87%. The test combination of 24-hour monitoring and motility studies can select patients with an accuracy of 91% who have an increase in esophageal exposure to gastric juice because of a deficient cardia. Antireflux surgery is designed to reduce esophageal exposure to gastric juice in patients with a deficient sphincter by creating a mechanical antireflux mechanism at the cardia. Therefore it is necessary to determine the mechanical status of the sphincter with manometry before surgery in such patients. Thus the indications for antireflux surgery are (1) uncontrolled symptoms of increased esophageal exposure to gastric juice; (2) a documented increase in esophageal exposure to gastric juice by 24-hour pH monitoring; and (3) a mechanically defective sphincter on motility with a pressure of 6 mm Hg or less, an overall length of 2 cm or less, and an abdominal length of 1 cm or less.