To analyze postprandial gastroesophageal reflux by means of ambulatory gastroesophageal pH monitoring for 24 h, four groups were studied prospectively: group A: 22 healthy volunteers; group B: 31 consecutive patients undergoing medical treatment for gastroesophageal reflux, group C1: 20 consecutive patients with symptomatic reflux awaiting surgical treatment by means of Nissen fundoplication (pre-Nissen evaluation) and group C2: group C1 patients reevaluated 6 months postoperatively (post-Nissen evaluation). Gastro-esophageal pH, as a measure of post-prandial reflux following the main meal of the day was evaluated by the Kaye test. In groups B, C1 and C2, esophageal manometry was also performed. Gastroesophageal pH monitoring revealed significant qualitative as well as quantitative differences in postprandial gastroesophageal reflux experienced by healthy subjects (group A) and surgically treated patients (group C2) compared to patients with pathologic reflux (groups B and C1). The postprandial reflux was significantly more acid and more important (Kaye's test value) in groups C1 and B than in groups A and C2. There were no differences in postprandial reflux between healthy subjects and patients treated by Nissen fundoplication (group C2). Only the pressure and length of the lower esophageal sphincter (LES) showed differences after Nissen fundoplication. We conclude that patients with pathologic reflux have more severe postprandial reflux than normal subjects; Nissen fundoplication corrects the degree of postprandial reflux to a normal range by elevating the LES pressure (11.3 x 1.4 vs. 22.4 +/- 1.6 mm Hg; p < 0.001) and length (2.7 +/- 0.2 vs. 3.7 +/- 0.1 cm; p < 0.001) in our patients.