In order to study the natural history of fundic gland polyposis, 23 patients with familial adenomatosis coli/Gardner's syndrome were examined over a follow-up period ranging from 17 mo to 13 yr (average 6 yr). Examinations included gastric radiography and endoscopy with biopsy. Fundic gland polyps were found in 10 individuals. The size and number of polyps varied considerably. During the follow-up period, there was an increase in number or size of polyps, or both, in 5 patients (aged 8-27 yr), a decrease or disappearance in 2 patients (aged 36 and 41 yr), an initial decrease or disappearance followed by a late-occurring increase in 2 patients (aged 28 and 35 yr), In addition, malignant or adenomatous changes of fundic gland polyps were not observed in any patient. Therefore, fundic gland polyposis in patients with familial adenomatosis coli/Gardner's syndrome may appear as early as 8 yr of age. In some patients there is a gradual increase in number and size of polyps, whereas in others, polyp proliferation ceases and polyps may even decrease in number and size. Our findings indicate that the fundic gland polyposis does not require prophylactic surgery and that careful periodic follow-up should suffice.