GERD is characterized by excessive esophageal acid exposure time. This suggests that either the rate of gastroesophageal reflux (GER) is higher and/or that the esophageal acid clearance time is longer. Transient LES relaxation (TLESR) is the single most common mechanism underlying GER in both normal subjects and patients with GERD. Whether or not the rate of TLESRs is higher in patients with GERD remains unclear. It is in the sitting or upright position that acid reflux mainly occurs, however, there seems to be no difference in the rate of TLESRs between both groups. The rate of TLESRs accompanied by acid reflux has been consistently shown to be significantly greater in patients with GERD than in normal subjects. Other mechanisms of reflux in patients with severe GERD are a hypotensive LES and ineffective esophageal motility which is found in severe GERD and which impairs bolus clearance of acid and thus increases acid contact time with the esophageal mucosa.