Hereditary haemorrhagic telangiectasia (HHT) is a dominantly inherited disease characterized by telangiectatic lesions. The disease manifestations are variable in severity. Early death due to complications has been described. We report an investigation of the prevalence and mortality of HHT in a Danish population, based on two cross-sectional surveys in combination with a long-term follow-up study. The prevalence of HHT in the county of Funen was 13.8 per 100,000 January 1. 1974 and 15.6 per 100,000 January 1. 1995. In the HHT group as a whole we found a slightly increased mortality, however among the HHT patients younger than 60 years at inclusion the mortality of HHT patients was twice the expected. The excess mortality was fully explained by severe HHT symptoms contributing to death. Future research should aim at identification of HHT patients at particular risk of developing severe complications.