An autosomal dominant syndrome caused by pathogenic variants in the SMAD4 gene, characterized by the combined features of juvenile polyposis syndrome (JPS) and hereditary hemorrhagic telangiectasia (HHT). JPS features include multiple juvenile polyps in the gastrointestinal tract and an increased risk of gastrointestinal cancers. HHT features include arteriovenous malformations (AVMs) and telangiectasias. The disease is juvenile polyposis/hereditary hemorrhagic telangiectasia syndrome (Monarch Disease Ontology identifier MONDO_0008278). Also known as: SMAD4-related juvenile polyposis/hemorrhagic telangiectasia syndrome.