This paper reviews the reported clinical, functional and biochemical abnormalities associated with abnormal fibrinogens. It appears from studies on fibrinogen Detroit that a single amino acid substitution in a critical part of the molecule can lead to major functional abnormalities and clinical consequences of either bleeding or thrombosis. Functional defects observed include: abnormal release of fibrinopeptide A or B or both after incubation with thrombin, abnormal polymerization and abnormal clot stabilization.