Hereditary haemorrhagic telangiectasia (HHT) is an autosomal dominant vascular disorder that is characterized by frequent nosebleeds, mucocutaneous telangiectases and vascular malformations that cause recurrent haemorrhage and arteriovenous shunting. Linkage analyses in one kindred identified an HHT locus on the long arm of chromosome 9 (maximum multipoint lod score = 6.20 between D9S60 and D9S61). Analyses in two other unrelated HHT families demonstrated that the disease in one was not linked to the locus on chromosome 9q3. We conclude that HHT is a genetically heterogeneous disorder. Based on its map location (9q3) and expression in vascular tissues, type V collagen is a possible candidate gene for HHT.