We analyzed somatic mutations of the adenomatous polyposis coli (APC), p53, and K-ras genes in gastroduodenal polyps and normal gastroduodenal mucosa from 21 familial adenomatous polyposis patients, using PCR-single-strand conformation polymorphism and direct sequencing methods. Seventy-five polyps were obtained from these patients endoscopically or surgically, and they were histopathologically diagnosed as mild adenoma, moderate adenoma, severe adenoma, adenocarcinoma, and fundic gland polyp. Examining the APC-coding region where somatic mutations in colorectal tumors are known to be clustered, we detected 47 somatic mutations. The frequency of mutation detected was 6 of 9 (67%) in ampullary adenomas, 1 of 2 (50%) in ampullary adenocarcinoma, 11 of 24 (46%) in non-ampullary adenomas, 26 of 29 (90%) in gastric adenomas, and 3 of 11 (27%) in gastric fundic gland polyps. These mutations frequently occurred at codons 1450, 1462-1465, and 1554-1556, the third being a newly found hot spot. All mutations formed stop codons that resulted in truncated APC proteins. K-ras mutation was detected only in an ampullary adenocarcinoma, and p53 mutation was not detected in any of the tumors analyzed. There was no somatic mutation detected in samples of flat mucosa that were diagnosed as normal mucosa both endoscopically and histopathologically. Frequent APC mutations in mild and small adenomas, similar to the findings in severe and large adenomas, suggested that the genetic change in the APC gene occurs in an early stage of forming gastroduodenal adenomas. Moreover, the presence of somatic APC mutations in fundic gland polyps suggests that inactivation of the APC gene plays a role not only in forming adenomas but also in forming hyperplastic polyps in fundic gland mucosa, and there may be some additional steps to the adenoma-carcinoma sequence.