To assess the effect of biliary diversion on gastric colonization by Campylobacter pylori, we undertook a retrospective histologic study of 24 patients with symptomatic bile reflux after peptic ulcer surgery, who had endoscopic gastric biopsies performed before and after a Roux-en-Y operation. The time interval between the preoperative and postoperative endoscopic examinations ranged from 0.8 to 9.8 yr (mean 4.7 yr). The partial gastrectomy specimen, which had been resected at the initial operation, was available for assessment in 12 patients (50%). Biopsy specimens were assessed for the presence of C. pylori and scored for severity of reflux gastritis by the use of a histologic grading system. Ten of the 12 partial gastrectomy specimens (83%) were C. pylori-positive. Only 13 of the 24 patients (54%) were C. pylori-positive before the Roux-en-Y operation, rising to 22 (92%) after biliary diversion (p = 0.008). The median reflux score was 6 in the partial gastrectomy specimens; it rose to 11 before the Roux-en-Y operation and fell again to 6 after biliary diversion (p less than 0.001). These results suggest that C. pylori may recolonize the gastric remnant after biliary diversion.