Hereditary hemorrhagic telangiectasia (HHT) or Osler-Rendu-Weber (ORW) disease is an autosomal dominant vascular dysplasia. Initial linkage studies identified an ORW gene localized to 9q33-q34 but with some families clearly excluding this region. A probable correlation in clinical phenotype between the 9q3-linked families and unlinked families was described with a significantly lower incidence of pulmonary arteriovenous malformations observed in the unlinked families. In this study we examined four unrelated ORW families for which linkage to chromosome 9q33-q34 has been previously excluded. Linkage was established for all four families to markers on chromosome 12, with a combined maximum lod score of 10.77 (theta = 0.04) with D12S339. Mapping of crossovers using haplotype analysis indicated that the candidate region lies in an 11-CM interval between D12S345 and D12S339, in the pericentromeric region of chromosome 12. A map location for a second ORW locus is thus established that exhibits a significantly reduced incidence of pulmonary involvement.