Inherited qualitative abnormalities of fibrinogen have been documented in 144 families. These dysfibrinogenemias have been inherited as autosomal dominant traits and usually are clinically silent, but in some cases are associated with bleeding, thrombosis, or defective wound healing. Dysfibrinogenemias may be associated with defects in any of the three basic steps in the thrombin-fibrinogen reaction, i.e., cleavage of the fibrinopeptides by thrombin, polymerization, and fibrin cross-linking. Biochemical studies of several abnormal fibrinogens have demonstrated that the functional defects are the result of single amino acid substitutions. Most of the reported cases may be distinguished by functional criteria and by the physicochemical behavior and biochemical nature of the abnormal protein.