Upper gastrointestinal polyps have been considered an uncommon finding in patients with Gardner's syndrome and familial polyposis coli. Investigators from Japan, however, have recently reported finding gastric and duodenal polyps in a high percentage of Japanese patients with these conditions. We endoscopically examined 11 affected members of the originally described Gardner's syndrome kindred to determine if upper gastrointestinal polyps also occurred in patients with Gardner's syndrome living in the United States. Six of the patients were found to have numerous small polyps of the gastric fundus and body. Polyp histology in 5 of the 6 patients was consistent with fundic gland hyperplasia. Biopsy specimens from the remaining patient demonstrated normal mucosa only. Another patient with no fundic polyps had a single antral polyp that was an adenoma. Eight patients exhibited small polyps of the duodenum. Biopsy specimens were obtained in 7 of the 8. All polyps biopsied were adenomas. The terminal ileum was examined by endoscopy in 9 of the 11 study patients. All 9 had ileal polyps, but the polyps were adenomas in 6 patients and lymphoid aggregates in 3 patients. The results indicate that upper gastrointestinal polyps are a common pleiotropic manifestation of the genetic defect responsible for Gardner's syndrome.