Objective: Fibrinogen plays an important role in the physiology of blood coagulation, angiogenesis, inflammation, platelet physiology and cell proliferation. Fibrinogen is a 340 kDa glycoprotein which is composed of six polypeptide chains. Hereditary dysfibrinogenemia is a disease wherein an inherited abnormality in the fibrinogen molecule results in defective fibrin clot formation. Methods: Fibrin polymerization was measured by turbidimetrical method. Scanning electron microscopy (SEM) was performed on Vega Plus TS 5135. Gene sequencing was performed by dideoxysequencing method. Molecular modelling was performed with DeepView/Swiss-PDB Viewer and 3D- mol Viewer and pdb2fib.ent data file. Results: The patient, a 25 year old woman, was admitted to our hospital with deep venous thrombosis and pulmonary embolism. Coagulation tests revealed prolonged thrombin time and normal fibrinogen level. Fibrin polymerization was impaired. The patient bears a heterozygous point mutaion in exon 9 of FGG causing substitution of 363 Tyr to Asn. SEM showed an abnormal fibrin clot. Conclusion: The patient's point mutation in the fibrinogen chain may be the direct cause of her deep venous thrombosis and pulmonary embolism. Tyr 363 plays a significant role in physiology of blood coagulation, especially during protofibril formation. Substitution of 363 Tyr to Asn changes the conformation of the interacting site and makes a new hydrogen bond between fibrinogen molecules. This leads to impaired polymerization, abnormal coagulation and fibrin clot formation. References: Okumura N., Gorkun O.V., Lord S.T. 1997. J. Biol. Chem. 272, 29596 – 29601. Acknowledgement: This work was supported by a grant of The Ministry of Health Czech Republic number 2373601.