Currently available methods to assess reflux of duodenal contents into the esophagus are cumbersome, unphysiologic, and inaccurate. The role of intestino-esophageal reflux has therefore been controversial. We assessed intestino-esophageal reflux using a new system which allows prolonged intraesophageal measurement of bilirubin, the major pigment of bile. Measurements were made with a newly developed fiber-optic sensor electrode connected to a portable data processing unit (BILITEC 2000, Synectics Medical Inc., Sweden). Light absorption was measured at the absorption peak of bilirubin and a reference point. Studies were performed in 9 subjects without esophagitis, 9 subjects with esophagitis and primary reflux disease and 7 subjects with erosive esophagitis after a total or subtotal gastrectomy. The fiberoptic electrode was placed 5 cm above the lower esophageal sphincter. In vitro studies showed linear correlations between absorbance measurements obtained with the BILITEC-unit and known bilirubin and bile acid concentrations, respectively (p < 0.01). Compared to both other groups, light absorption was markedly increased in the subjects who had esophagitis after a total or subtotal gastrectomy (p < 0.05) indicating severe biliary reflux. An increase in bilirubin absorption occurred particularly during the post-prandial and supine periodes (p < 0.01). A Roux-en-Y biliary diversion procedure completely abolished bile reflux in 2 of these patients. These data indicate that ambulatory 24-hour fiberoptic measurement of bilirubin in the esophagus is feasible and allows quantitation of intestino-esophageal reflux. Intestino-esophageal reflux occurs particularly during the postprandial period and the early morning hours in patients who had a previous subtotal or total gastrectomy.