The changes produced by the Nissen fundoplication were measured in 12 patients, who required surgery to control their reflux oesophagitis. The gastro-oesophageal junction of each patient was studied before and three months after operation by station pullthrough manometry and prolonged pH monitoring of the distal oesophagus. All patients were free from reflux symptoms post-operatively. The Nissen fundoplication resulted in a significant increase in the pressure, but not the length, of the lower oesophageal high pressure zone. A greater proportion of this zone was situated in the abdomen postoperatively. Prolonged pH monitoring showed a significant improvement in all the measured indices of acid reflux. Nissen fundoplication restores competence to the gastro-oesophageal junction as judged by manometry and pH monitoring. This kind of study should be performed to document the efficiency of other anti-relfux procedures.