The autosomal dominantly inherited juvenile polyposis syndrome (JPS) leads to the development of multiple hamartomatous polyps in the gastrointestinal tract and is a precancerous condition. In a large family with a newly identified SMAD4 mutation (c.543delC), we describe the clinical manifestations of JPS. Nine affected SMAD4 mutation-positive family members were screened and treated for manifestations of JPS. Two family members were symptomatic at the time of diagnosis; seven were asymptomatic - independent of the severity of the manifestation. Each mutation carrier presented with colonic juvenile polyps, seven out of nine with additional gastric manifestations. One asymptomatic patient had early gastric cancer; another patient had a villous adenoma with high-grade intraepithelial neoplasia in the colon. Three patients had biliary lesions including a bile duct hamartoma in one and gallbladder polyps in two. Three patients had gastrointestinal vascular malformations. All mutation carriers were affected by JPS. Interestingly, the manifestations and their severity differed considerably between the patients, suggesting secondary factors influencing JPS manifestations such as Helicobacter pylori infection.