Inherited qualitative abnormalities of fibrinogen have been documented in more than 100 families. These dysfibrinogenemias usually are clinically silent, but in some cases are associated with bleeding, thrombosis, or defective wound healing. Abnormalities of the fibrinogen molecule may impair any of the major steps involved in the conversion of fibrinogen into stabilized fibrin; i.e., cleavage of the fibrinopeptides by thrombin, polymerization, and cross-linking of fibrin. Biochemical studies of several abnormal fibrinogens have demonstrated that the functional defects are the result of single amino acid substitutions. The hereditary dysfibrinogenemias are the first coagulation disorder in which the pathophysiology has been elucidated on a molecular level. Studies of these "experiments of nature" have important implications in such diverse processes as wound healing and thrombosis.