A young man with hereditary juvenile polyposis, a duodenal adenoma with carcinoma in situ, and multiple arteriovenous malformations of the lungs and liver is reported. Because of the risk of malignancy, it is suggested that patients with juvenile polyposis have endoscopic or radiographic surveillance of the upper gastrointestinal tract in addition to either prophylactic colectomy or regular colonoscopic surveillance with polypectomy. In view of the serious potential complications of associated pulmonary, cerebral, and hepatic arteriovenous malformations, screening of patients with juvenile polyposis with chest radiography, magnetic resonance imaging of the brain, and sonography of the liver may be warranted.