Two unrelated large sibships, including 10 cases of congenital afibrinogenemia among 27 sibs, are reported. Both sibships were the product of uncle-niece marriages. They were not selected for any particular clinical manifestation and should provide some information on genetic fitness. Six of the patients died in childhood, two affected boys are adolescent and two affected patients are young women. Two of the four survivors had spontaneous ruptures of the spleen. Fitness in this very rare disease seems to be close to zero and the inheritance is autosomal recessive.