Gastroesophageal reflux (GER) can develop in patients with esophageal achalasia either before treatment or following pneumatic dilatation or Heller myotomy. In this study we assessed the value of pre- and postoperative pH monitoring in identifying GER in patients with esophageal achalasia. Ambulatory pH monitoring was performed preoperatively in 40 patients with achalasia (18 untreated patients and 22 patients after pneumatic dilatation), 27 (68%) of whom complained of heartburn in addition to dysphagia (group A), and postoperatively in 18 of 51 patients who underwent a thoracoscopic (n=30) or laparoscopic (n=21) Heller myotomy (group B). The DeMeester reflux score was abnormal in 14 patients in group A, 13 of whom had been treated previously by pneumatic dilatation. Two types of pH tracings were seen: (1) GER in eight patients (7 of whom had undergone dilatation) and (2) pseudo-GER in six patients (all 6 of whom had undergone dilatation). Therefore 7 (32%) of 22 patients had abnormal GER after pneumatic dilatation. Postoperatively (group B) seven patients had abnormal GER (6 after thoracoscopic and 1 after laparoscopic myotomy). Six of the seven patients were asymptomatic. These findings show that (1) approximately one third of patients treated by pneumatic dilatation had GER; (2) symptoms were an unreliable index of the presence of abnormal GER, so pH monitoring must be performed in order to make this diagnosis; and (3) the preoperative detection of GER in patients with achalasia is important because it influences the choice of operation.