Fatty foods have been identified as precipitating factors in symptomatic gastroesophageal reflux (GER). A fat meal has also been found to decrease lower esophageal sphincter pressure (LESP) in normal subjects. We used the ambulatory 24-h pH monitor to assess esophageal acid exposure in 10 normal subjects and 10 GER patients following low and high fat meals eaten in two body positions. The meals had nearly identical protein content, volumes, and calories. On successive days, patients ingested one of the meals twice, followed by random assignment to 3 h upright and 3 h recumbent position. Acid exposure for each hour over a 3-h postprandial (PP) period was assessed as the percent time pH less than 4.0. Increased upright acid exposure occurred in normals after the high fat (6.2 +/- 2.1%; mean +/- SE) compared with the low fat meal (1.5 +/- 0.5%; p less than 0.05). GER patients had greater (p less than 0.05) acid exposure than normals in all study periods, but no differences were found between low and high fat meals in either study position. High fat meals induce upright GER in normals, but do not significantly affect the abnormal amount of GER in patients. In addition, progressive increases in acid exposure were found over the 3 postprandial hours in GER patients in a recumbent position. The findings are consistent with prior data showing decreased LESP with a fat meal in normals.