BACKGROUND: Previous studies suggest an association between obesity and oesophageal (OA) and oesophagogastric junction adenocarcinomas (OGJA). However, these studies have been limited in their ability to assess whether the effects of obesity vary by gender or by the presence of gastro-oesophageal reflux (GERD) symptoms. METHODS: Individual participant data from 12 epidemiological studies (8 North American, 3 European and 1 Australian) comprising 1997 OA cases, 1900 OGJA cases and 11 159 control subjects were pooled. Logistic regression was used to estimate study-specific odds ratios (ORs) and 95% confidence intervals (CIs) for the association between body mass index (BMI, kg/m(2)) and the risk of OA and OGJA. Random-effects meta-analysis was used to combine these ORs. We also investigated effect modification and synergistic interaction of BMI with GERD symptoms and gender. RESULTS: The association of OA and OGJA increased directly with increasing BMI (P for trend <0.001). Compared with individuals with a BMI <25, BMI >/=40 was associated with both OA (OR 4.76, 95% CI 2.96-7.66) and OGJA (OR 3.07, 95% CI 1.89-4.99). These associations were similar when stratified by gender and GERD symptoms. There was evidence for synergistic interaction between BMI and GERD symptoms in relation to OA/OGJA risk. CONCLUSIONS: These data indicate that BMI is directly associated with OA and OGJA risk in both men and women and in those with and without GERD symptoms. Disentangling the relationship between BMI and GERD will be important for understanding preventive efforts for OA and OGJA.