The purpose of the present study was to investigate the relationship of the gastroesophageal pressure gradient (GEPG) to lower esophageal sphincter pressure (LESP) in normal and in severely obese subjects. Eight lean volunteers with no clinical evidence of gastroesophageal reflux and eight asymptomatic severely obese patients (at least 80% over their ideal weight) underwent esophageal manometric studies with measurements of the LESP and GEPG in both inspiration and expiration. The LESP/GEPG ratio was also calculated in both inspiration and expiration. Acid sensitivity was assessed by means of infusion of 0.1 N HCl subsequent to the baseline motility study. There was no significant difference between the LESP in obese patients (O.P.) and normal subjects (N.Sb.) in either inspiration (mean +/- SEM in mm Hg: N.Sb. = 16.4 +/- 1.6, O.P. = 18.7 +/- 2.5), or expiration (N.Sb. = 16.6 +/- 1.5, O.P. = 20.6 +/- 2.6). However, the GEPG in both inspiration (N.Sb. = 13.3 +/- 1.6, O.P. = 23.1 +/- 2.0; p less than 0.001) and in expiration (N.Sb. = 2.1 +/- 0.5, O.P. = 8.1 +/- 1.1; p less than 0.001) was significantly higher in obese patients than in controls. As a result, the GEPG/LESP ratios were also higher (expiration N.Sb. = 0.15 +/- 0.03, O.P. = 0.46 +/- 0.10; p less than 0.01) in obese patients; and for inspiration (N.Sb. = 0.86 +/- 0.13, O.P. = 1.33 +/- 0.12; p less than 0.01) in the obese patients the ratio was greater than 1. None of the normal subjects exhibited acid sensitivity, but 6 of the 7 obese patients tested developed heartburn during acid infusion. In conclusion, the GEPG/LESP ratio in inspiration was greater than unity for obese patients inspite of normal LESP. Such a change in the ratio could facilitate reflux in obese patients.