Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant bleeding disorder characterized by localized angiodysplasia. Mutations in either of two genes, endoglin or ALK-1, can cause HHT. Both genes encode putative receptors for the transforming growth factor-beta superfamily of ligands. Many mutations in each gene have been identified in HHT kindreds from around the world, and with few exceptions mutations are unique and family specific. The prevalence of HHT in the Leeward Islands of the Netherlands Antilles is possibly the highest of any geographical location. We wished to establish whether this high prevalence is due to a genetic founder effect or to multiple mutational events. HHT kindreds from the Netherlands Antilles and The Netherlands were screened for mutations in the two genes associated with HHT. Haplotype analysis of a 5-cM region on chromosome 9 flanking the endoglin gene revealed three distinct disease haplotypes in the ten Antillean families studied. Seven of these families share a splice-site mutation in exon 1 of endoglin. Two other Antillean families share a missense mutation in exon 9a of endoglin. This mutation was also found in a Dutch family that shares the same disease haplotype as the Antillean families with this mutation. Thus it appears that HHT in the Netherlands Antilles is due to a limited number of ancestral mutations in the endoglin gene, and that one of these mutations was introduced into the African slave population by a Dutch colonist. The limited scope of mutations suggests that a presymptomatic screening program for HHT would be feasible in this population.