Nineteen patients with abnormal gastroesophageal reflux (13 with and six without hiatal hernia) had esophageal manometry and simultaneous 24-hour monitoring of esophageal pH and intra-abdominal pressure (IAP). Only 8% of all IAP challenges induced a reflux episode. This incidence increased to 13% in patients with a distal esophageal sphincter (DES) pressure of less than 5 mm Hg and an abdominal esophageal length of less than 1 cm, whereas it was only 6% in patients with a greater DES pressure and a longer abdominal esophagus. At the same time, there was an average of 2.7 reflux episodes per hour, of which 38.7% were caused by a challenge of IAP indicating that other mechanisms, besides changes in abdominal pressure, can cause reflux. There was no difference in DES pressure, length of abdominal esophagus, and the effect of IAP challenges in patients with and those without a hiatal hernia.