Primary pulmonary hypertension (PPH) is a serious disease of unknown cause in which small arteries in the lungs become obstructed. Mean survival is less than three years, and women develop PPH twice as commonly as men. It is familial (FPPH) in about 6 percent of cases. The National FPPH Registry was established in 1994 to collect and analyze family history and clinical data from PPH families to better characterize the disease phenotype as well as to identify the underlying genetic etiology. Through the collection of 72 families, FPPH has been shown to be inherited as an autosomal dominant disorder, with incomplete penetrance and genetic anticipation. Micro-satellite marker studies in six families have identified linkage to chromosome 2q31 without evidence of genetic heterogeneity.