Distension of the esophagus can cause heartburn and chest pain; however, none of the available techniques to study the esophagus measure esophageal distension. We evaluated the technique of high-frequency intraluminal ultrasound probe (HFIUS) to measure the esophageal cross-sectional area (CSA) during gastroesophageal reflux (GER). The following methods were used: 1) the CSA of agarose gel tubes of known dimensions were measured using ultrasound probes; 2) seven normal subjects were studied to evaluate the esophageal CSA during different bolus volumes (1, 5, 10, 15, and 20 ml) of water swallows (WS); and 3) simultaneous pressures, pH, and ultrasound images of the esophagus were recorded in healthy subjects. In vitro studies showed that the HFIUS measured the CSA of the tubes accurately. The maximal CSA of the distal esophagus during WS with boluses of 1, 5, 10, 15, and 20 ml were 54, 101, 175, 235, and 246 mm², respectively. Esophageal contents during 62 episodes of transient lower esophageal sphincter relaxations, 29 pH positive, and 33 pH negative GER episodes revealed that reflux of air into the esophagus occurred more frequently than liquid. The median CSA and estimated diameter of the esophagus during liquid GER was 44.1 mm² and 7.5 mm, respectively. We conclude that HFIUS is a valid technique to measure the CSA of the esophagus in vivo during GER. Distension of the esophagus during physiological GER is relatively small.