A total of 26 patients with familial polyposis coli without extracolonic manifestations were examined by gastroduodenoscopy. Histologically verified polyps were found in 18 patients (69 percent, 95 percent confidence limits 48-86). Gastric adenoma was diagnosed in one patient, fundic gland polyposis in six patients, and duodenal adenomas in 12 patients. It is concluded that the incidence of gastroduodenal polyps is independent of the clinical presence of other extracolonic manifestations. It is advisable that polyposis patients should be followed with gastroduodenoscopy and biopsy of polyps.