This study evaluated the effect of the 270 degrees Belsey Mark IV fundoplication on 24 hour ambulatory pH metry variables. Thirty seven patients with confirmed gastro-oesophageal reflux disease who had a Belsey Mark IV antireflux procedure were evaluated preoperatively and three to six months postoperatively including endoscopy, lower oesophageal sphincter manometry, and 24 hour ambulatory pH metry. In 30 of 37 patients the Belsey Mark IV fundoplication was judged successful based on symptom relief and healing of oesophagitis. In these 30 patients percentage reflux for total, upright, and supine time (median and range) decreased significantly (p < 0.001) from 10.0% (2.7-35.3%), 10.8% (3.2-39.9%), and 6.7 (0.0-33.0%) respectively to 0.5% (0.0-7.4%), 0.6% (0.0-13.7%), and 0.1% (0.0-4.9%) after operation. This decrease in reflux time resulted predominantly from a significant (p < 0.001) reduction in the number of reflux episodes from 98 (23-231) to 14 (0-82) postoperatively. Normalisation of total reflux time (upper limit of normal for time with pH below 4:4%) was found in 89% and normalisation of total and upright and supine reflux in 64% of successfully operated patients with confirmed abnormal acid reflux before operation. A successful antireflux procedure was associated with significant (p < 0.001) increases in lower oesophageal sphincter pressure from 7.8 (0.6) mm Hg to 14.5 (0.7) mm Hg mean (SEM). In seven patients with a failed antireflux operation basal lower oesophageal sphincter pressure did not change significant;y (preoperative value 5.7 (1.3) mm Hg; postoperative value 7.8 (0.8) mm Hg). In these patients reflux time did not decrease after the operation and remained in the abnormal range in all patients. It is concluded that a successful, in contrast with a failed, Belsey Mark IV fundoplication is associated with significant decreases in total, upright, and supine reflux time. Normalisation of pH metry variables is not a prerequisite for successful antireflux surgery. A surgery. A successful 270 degree Belsey Mark IV fundoplication was associated with a significant increase in basal lower oesophageal sphincter pressure, in contrast with the failure group. Restoration of a sufficient lower oesophageal sphincter barrier is an important aim in antireflux surgery.