HHT is an autosomal dominant disease characterised by diffuse muco-cutaneous and visceral telangiectases in potentially all organs. Mutations in two different genes identify HHT type 1 and HHT type 2: endoglin located on chromosome 9q33-q34 and ALK-1 or ACVRL1 on chromosome 12q13, respectively. The existence of a third locus has also been hypothesised. HHT-1 is considered a more severe form of the disease with an earlier onset of epistaxis and telangiectases and a higher prevalence of pulmonary arteriovenous malformations than that found in HHT-2 subjects. Usually, a typical HHT patient has epistaxis, muco-cutaneous telangiectases and GI bleeding in later life, even though this clinical scenario represents only one of the possible HHT patterns. In fact, vascular malformations often remain silent until the onset of a severe complication, which frequently is the first clinical manifestation of HHT. The lung and brain are of particular concern because each may contain clinically silent lesions that can result in sudden morbidity and mortality. At present, awaiting the availability of genetic testing, only an expert in the clinical patterns and diagnostic imaging of HHT can permit a definite diagnosis in individuals at high risk for the disease.