Although there have been isolated reports of an acquired abnormal fibrinogen in patients with liver disease, its frequency and clinical significance is not known. In this study 121 consecutive patients with a wide spectrum of hepatic disorders were screened for abnormal fibrin polymerization. A simple colorimetric method using Reptilase was employed. Of 32 patients with proven cirrhosis, 16 (50%) showed abnormal fibrin polymerization. The incidence in decompensated alcoholic cirrhosis was particularly high. The abnormality was also detected in all patients with acute liver failure and seven of 15 with chronic active liver disease. Clinical improvement often correlated with its disappearance. Two patients with primary liver cell tumours demonstrated the abnormal polymerization. In patients with bleeding oesophageal varices the detection of abnormal fibrin polymerization was associated with a poor prognosis. None of the patients with surgical obstructive jaundice (26) or miscellaneous liver disorders (37) had abnormal fibrin polymerization. The occurrence of abnormal fibrin polymerization in liver disease is more frequent than previously suspected and usually signifies severe primary hepatocellular dysfunction. Evidence is presented to support the presence of a primary abnormality of fibrinogen as the cause of impaired fibrin monomer polymerization.