Hereditary haemorrhagic teleangiectasia (HHT) is an autosomal dominantly inherited disorder characterised by cutaneous and mucosal telangiectasias, epistaxis and arteriovenous malformations in lung, liver, central nervous system and gastrointestinal tract. Mutations in the genes for endoglin (ENG) and for activin A receptor type II-like kinase 1 (ACVRL1) have been identified to cause HHT. We performed molecular diagnosis in clinically affected probands of 52 HHT families and detected mutations in 34 cases. We report on a total of 19 novel disease-causing mutations, 7 in ENG and 12 in ACVRL1. Three of the novel mutations affected acceptor splice-sites in the ENG gene. RNA analyses in these three patients and in two further patients described before resulted in reduction of the transcript or in a shortened transcript. Furthermore, we identified a family with the mutation c.199C>T in the ACVRL1 gene with liver AVMs. This is the fifth family with this mutation and liver AVMs, clearly indicating a genotype-phenotype correlation for this mutation.