Hereditary haemorrhagic telangiectasia (HHT, Rendu-Osler-Weber syndrome) exemplifies an important group of diseases which have catalysed advances in the understanding of fundamental pathophysiological mechanisms. In this paper areas of clinical management of HHT are discussed and the molecular pathogenesis is reviewed. The first section is aimed at all clinicians and concentrates on the recognition of a disorder in which silent cerebral and pulmonary involvement may be life threatening if left untreated. Recent data concerning the diagnostic and treatment modalities for pulmonary arteriovenous malformations (PAVMs) are also reviewed, and the growing concern that many patients with HHT may have small or residual PAVMs is highlighted. The paucity of good longitudinal data on these patients and others with different forms of HHT highlights the need for further clinical studies. In the second section the results of molecular research which suggests a role for receptors and ligands of the transforming growth factor (TGF)-β superfamily in the pathogenesis of this vascular disease are discussed. The means by which such information may relate to the clinical heterogeneity observed in HHT are specifically addressed, and more fundamental questions such as how reduced cell surface expression of endoglin predisposes a patient to develop PAVMs are also discussed.