Thirty-nine patients with factor XI deficiency were diagnosed in our general hospital, which serves all the French Basque Country (about 290 000 inhabitants), between 1985 and 1995. Biological and clinical data of these cases are reported herein. Factor XI deficiency seems significantly more frequent in Basques than in the other non-Ashkenazi populations. Molecular studies should be performed for typing these cases in the view of population genetics.