Hereditary haemorrhagic telangiectasia (HHT), also known as Rendu-Osler-Weber syndrome, is an inherited multisystemic disorder with recurrent epistaxis, mucocutaneous telangiectasia and visceral arteriovenous malformations. The purpose of this study is to provide data about multiple clinical aspects of HHT and responses to treatment. For comparison of some aspects also data of non-affected relatives is collected (second cohort). the questionnaire has been designed primarily for web based entry, but can also be circulated in paper format on request.