OBJECTIVE: To compare the competence of the lower oesophageal sphincter after Nissen fundoplication with that in a control group of healthy volunteers using a new computerized manometric device that integrates the pressure and length of the entire sphincter into one measurement, called the vector volume. DESIGN: Open study. SETTING: University hospital, Italy. SUBJECTS: 18 patients, 6-52 months (mean 10 months) after they had undergone Nissen fundoplication for gastro-oesophageal reflux, and 14 healthy volunteers. INTERVENTIONS: Oesophageal computerised manometric system comprising a catheter with eight side holes radially oriented to each other at 45 degrees on one level and 24-hour oesophageal pH monitoring. MAIN OUTCOME MEASURES: Differences in percentage time of oesophageal acid exposure, number of episodes of gastro-oesophageal reflux, and lower oesophageal sphincter vector volume. RESULTS: The pH of the oesophagus was returned to the normal range in all patients after the Nissen fundoplication. There was a significant reduction in both the percentage time of acid exposure and the number of episodes of gastro-oesophageal reflux compared with the control group (p < 0.001). The vector volume of the intra-abdominal part of the lower oesophageal sphincter was significantly increased compared with the control group (p < 0.04). Lower oesophageal sphincter pressure measured at the respiratory inversion point, and lower oesophageal length, both increased after fundoplication to become similar to control values. When sphincter pressure was measured as a mean of the sphincter pressure curve there was a clear difference postoperatively compared with controls (p < 0.02). CONCLUSIONS: Nissen fundoplication corrects abnormal gastro-oesophageal reflux but abolishes normal physiological postprandial reflux; this is caused by the gastric fundus that encircling the intra-abdominal oesophagus, restores sphincter pressure and length, and modifies its shape.