In order to evaluate the prevalence of duodenal adenomas in familial adenomatous polyposis (FAP) and the risk of carcinoma development, a multicenter study was initiated in Denmark, Finland, Holland, Norway and Sweden, which have national polyposis registers with an almost complete registration. Patients aged 20 years or more are being examined with biennial gastroduodenoscopy during 1990-2000. Multiple duodenal biopsies are examined by one pathologist from each country, and the endoscopic and histological criteria of Spigelman have been adopted. At the end of August 1992, 312 patients with a median age of 37 years (range 20-86) had completed their first endoscopy. The duodenum was examined in 310 patients, of whom 199 (64%) had duodenal adenomas. Twenty-two patients (11% of all patients with duodenal adenomas) had no endoscopically visible polyps. One patient had an asymptomatic adenocarcinoma. The Spigelman stage worsened significantly (P < 0.05) with time from the diagnosis of FAP, which may suggest an increasing risk of carcinoma by time.