To determine if hiatus hernia (HH) contributes to the delayed clearance of acid from the esophagus in patients with gastroesophageal reflux (GER), we performed simultaneous esophageal pH recordings and radionuclide studies in three study populations: 12 GER patients with HH, 5 GER patients with no HH, and 8 subjects with HH but no GER symptoms. Acid clearance was measured at 5 cm. above the manometrically located lower esophageal sphincter (LES) after injecting a 15-ml. bolus of 0.1 N HCl at 15 cm. above the LES. The acid was labeled with 200 mu Ci of 99mTc-sulfur colloid. Acid clearance was also measured at 10 cm. above the LES after injection of a 15-ml. bolus of 0.1 N HCl at 20 cm. above the LES. Acid clearance at 5 cm. above the LES was faster in GER patients with no HH compared to GER patients with HH and asymptomatic HH subjects. Acid clearance was faster at 10 cm. than 5 cm. above the LES in all HH and non-HH subjects studied. In non-HH subjects, each swallow resulted in an increase in pH (a monophasic pH response) at 5 and 10 cm. above the LES. In symptomatic as well as asymptomatic HH subjects, swallows resulted in an initial fall followed by a rise in pH at 5 cm. above the LES (a biphasic pH response). Radionuclide studies showed reflux of the isotope-labeled acid into the esophagus followed by clearance (a biphasic response) accompanying swallows in 15 of the 20 HH subjects. Swallow-induced reflux was not detected by radionuclide scanning in non-HH subjects. Based on these observations, we conclude that during acid clearance a small amount of acid is trapped in the HH sac and refluxes into the esophagus during subsequent swallows when there is relaxation of the LES, and these repeated episodes of acid reflux from the HH account for the delayed acid clearance observed in GER patients with HH.