The objective of this study is to determine if acid or bile reflux into the esophagus is responsible for refractory symptoms of gastroesophageal reflux disease (GERD) despite gastric acid suppressant therapy, including histamine type-2 receptor antagonists (H2RA) or proton pump inhibitors (PPI). This will be determined by simultaneously monitoring, over a 24 hour period, both esophageal pH and bilirubin concentrations in patients with GERD and to allow us to determine if acid reflux, bile refulux, both or neither is responsible.