The variables influencing the intraoperative measurement of lower oesophageal sphincter (LOS) pressure and a new method of assessing the effect of a fundal wrap on LOS function were investigated in 13 patients undergoing fundoplication. All patients had a 360 degrees wrap fashioned around a 50-Fr bougie; four also underwent highly selective vagotomy. The effect of the fundal wrap, independent of LOS pressure, was assessed by inducing LOS relaxation with balloon distension. Preoperative mean(s.e.m.) LOS pressure (14.1(2.6) mmHg) did not correlate with that at the start of operation (11.5(1.5) mmHg). After mobilization of the oesophagus mean(s.e.m.) LOS pressure increased significantly (to 19.8(2.0) mmHg; P < 0.0005) and oesophageal distension elicited LOS relaxation on 16 of 33 occasions. After completion of the wrap there was no further increase in LOS pressure, but LOS relaxation occurred after only five of 33 distensions (P < 0.01). These findings indicate that anaesthesia and intraoperative manipulation during fundoplication have significant effects on LOS function that detract from the validity of intraoperative assessment. They question the rationale of current approaches to intraoperative manometry for the assessment of antireflux surgery.