The natural history of gastric adenomas was studied in 26 patients with familial adenomatosis coli (FAC)/Gardner's syndrome. The patients were followed for an average of 6.8 years (1 year, 5 months to 15 years 4 months) and examined repeatedly using gastric radiography, endoscopy, and biopsy. Gastric adenomas were present in 13 (50%). During the follow-up period, there was a new appearance of gastric adenomas in six patients but no distinct change in number, size, and histologic features in the remaining seven patients. In a 51-year-old man, an antral adenoma that measured 13 mm in diameter and had an irregular central depression did not change in size or morphology during 4 years and 1 month, but malignant change was evident at the time of endoscopic biopsy. The findings indicate that in patients with this disease, gastric adenomas require careful repeated follow-up examinations using endoscopic biopsy and, if feasible, endoscopic removal is recommended.