Upper gastrointestinal tract polyps were sought prospectively using endoscopy and biopsy in 34 patients with familial adenomatosis coli belonging to 18 unrelated families. Gastric and/or duodenal polyps, usually small and multiple, occurred in 28 patients (82%). Histologically verified extracolonic adenomas were present in 19 patients (56%). Gastric adenomas, all in the antrum, and duodenal adenomas occurred in four (12%) and 16 (48%) patients, respectively. In one patient, a duodenal adenocarcinoma and a bile duct adenoma were also found, and one patient had an adenocarcinoma of the bile ducts. Multiple non-neoplastic polyps were found in 19 patients (56%), most often in the stomach and also in the duodenum in 12 patients; they co-existed often with adenomas. In addition, there were nine patients with ileal polyps, most of them showing lymphoid hyperplasia but also one with adenomas. It is suggested that familial adenomatosis affects the whole gastrointestinal tract, not only the colon and rectum as believed earlier. Although upper gastrointestinal tract adenomas are not as consistent and multiple as those in the colon, and probably do not require prophylactic surgery, regular lifelong endoscopic follow up is warranted because of obviously increased cancer risk.