An important concentration of patients with Rendu-Osler disease occurs in the Valserine valley of the French Jura. A study of marriages shows that, in spite of its somewhat remote location, the valley cannot be considered an isolate, but that, on the contrary, exogamy is widely practised. Only 17.8% of the genes of inhabitants during the present century can be traced to the original population, but persons affected with the disease belong to a subgroup of the population which has lived in the villages for more than 10 generations. All the patients in 85 sibships are related. The smallest number of originator couples who lived at the beginning of the eighteenth century amounts to 16, the unique originator may therefore have lived approximately four generations earlier. However, as other patients who lived at the periphery of the Valserine valley do not appear to be related to any patients living in the valley, and because there has been considerable immigration into the valley, a number of hypotheses to explain the distribution of the disease in the region remain possible.