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Six patients with congenital afibrinogenemia belonging to four kindreds were studied with regard to hereditary pattern. In two families the hereditary pattern appeared to be autosomal recessive; in the two other pedigrees, on the contrary, the pattern seems autosomal intermediate. In the first type, all family members, excluding the patients, showed normal fibrinogen levels; in the second type, family members could be divided into two groups: normal and heterozygotes. The heterozygotes had fibrinogen levels of 192±30 mg/dl, definitely lower than that of a normal control population. The average level of the normal relatives was 361±81,9 mg/dl, practically identical to that of a normal control group unrelated to the homozygotes. In the past these differences were thought to be secondary to variances in fibrinogen assays from one laboratory to the other. It now appears that they are real ones since they can be observed in the same laboratory using the same fibrinogen technique. It must be concluded that congenital afibrinogenemia shows two patterns of hereditary transmission, one autosomal recessive and the other autosomal intermediate.