The means by which fundoplication protects against reflux is disputed. We studied the resting and dynamic features of the lower esophageal sphincter (LES) and 24-hour pH monitoring in 26 patients before and after laparoscopic Nissen fundoplication. Resting features were LES pressure, abdominal length, and total length. Dynamic function was assessed by the residual pressure in the LES during a swallow measured as the bolus flowed though the LES. All patients experienced near-total relief of heartburn and all but one had normal postoperative acid scores. Resting LES characteristics were restored to normal. Mean residual pressure on swallowing was 7.1+/-3.2 mm Hg in the patients postoperatively compared with 1.2+/-1 mm Hg preoperatively and 4.0+/-2.4 mm Hg in normal subjects. Eighteen of 26 patients had residual LES pressure within the normal range (<8.2 mm Hg). There was a tendency for residual pressures to be lower as experience with the procedure was gained. Incomplete LES relaxation is not necessary for effective functioning of a Nissen fundoplication. In construction of a Nissen fundoplication, creating a large retroesophageal window and deliberate dissection of the back of the posterior fundus from the left crus allows the creation of an effective antireflux procedure with restoration of static LES parameters to normal and minimal limitation of LES relaxation.