Hereditary hemorrhagic telangiectasia (HHT) and familial juvenile polyposis (EJP) are two rare autosomal dominant disorders, Genetic heterogeneity has been shown for HHT and is likely for FJP as well. This paper describes the coexistence of both diseases in a girl and her father in addition to twelve members of five families and two sporadic cases reported in the literature. This implies a new phenotype which may be important in elucidating the underlying genetics in HHT and FJP. Clinical diagnosis of one disease should induce screening for symptoms of the other.