Hereditary haemorrhagic telangiectasia (HHT) is a genetically heterogeneous dominant disorder. Two disease loci have been mapped to chromosomes 9q3 and 12q. In a large pedigree, with an unusually high number of patients with liver vascular malformations, both previously mapped loci have been excluded. The loci for two other inherited vascular malformation diseases, cerebral cavernous malformations and multiple cutaneous and mucosal venous malformations, have also been excluded. Thus we conclude that at least a third, as yet unmapped, HHT locus does exist, possibly associated with high frequency of liver involvement.