The effect of meal volume and fat content on gastroesophageal reflux was investigated in 20 asymptomatic healthy subjects. In each subject, intraesophageal pH monitoring was performed during a 3-hr postprandial period (PP) in the same position (supine or upright) on two successive days. On day 1, 500-ml low- and high-fat meals were ingested and, on day 2, an 800-ml low-fat meal was ingested. The acid exposure time was assessed as the percentage of time with a pH < 4.0. The acid exposure time in subjects in the upright position was significantly longer in the 800-ml group than in the 500-ml group for the entire PP (2.7 +/- 1.5%; mean +/- SE, 0.7 +/- 0.4%; P < 0.05). Of subjects in the supine position, the high-fat group showed significantly longer acid exposure time than the low-fat group both for the entire PP (7.6 +/- 3.0%, 0.7 +/- 0.5%; p < 0.05) and for the second hour (P < 0.05). We have demonstrated that differences in the meal volume and fat content influence gastroesophageal reflux in healthy asymptomatic subjects and that this influence varies with the position.