BACKGROUND: Obesity has been associated with gastro-oesophageal reflux disease (GERD); however, the mechanism by which obesity may cause GERD is unclear. AIM: To examine the association between oesophageal acid exposure and total body or abdominal anthropometric measures. METHODS: A cross-sectional study of consecutive patients undergoing 24 h pH-metry was conducted. Standardised measurements of body weight and height as well as waist and hip circumference were obtained. The association between several parameters of oesophageal acid exposures and anthropometric measures were examined in univariate and multivariate analyses. RESULTS: 206 patients (63% women) with a mean age of 51.4 years who were not on acid-suppressing drugs were enrolled. A body mass index (BMI) of >30 kg/m(2) (compared with BMI<25 kg/m(2)) was associated with a significant increase in acid reflux episodes, long reflux episodes (>5 min), time with pH<4, and a calculated summary score. These significant associations have affected total, postprandial, upright and supine pH measurements. Waist circumference was also associated with oesophageal acid exposure, but was not as significant or consistent as BMI. When adjusted for waist circumference by including it in the same model, the association between BMI>30 kg/m(2) and measures of oesophageal acid exposure became attenuated for all, and not significant for some, thus indicating that waist circumference may mediate a large part of the effect of obesity on oesophageal acid exposure. CONCLUSIONS: Obesity increases the risk of GERD, at least partly, by increasing oesophageal acid exposure. Waist circumference partly explains the association between obesity and oesophageal acid exposure.