The coagulation inhibitor protein C was measured in 151 patients with various liver diseases. The protein C level was significantly decreased in patients with alcoholic cirrhosis (n = 73) compared to patients with steatosis (n = 24) (40 +/- 2%) vs. 88 +/- 4%, mean S.E., p less than 0.001). It was also decreased in cases of acute liver damage (n = 8) and in patients with non-alcoholic cirrhosis (n = 15) (35 +/- 7% and 36 +/- 4%, respectively). A significant correlation was found between protein C and Normotest, antithrombin, heparin cofactor II, (r = 0.83, r = 0.82, r = 0.81, respectively, p less than 0.001). There was also a significant correlation between protein C and serum concentrations of albumin (r = 0.61, p less than 0.001), but a negative association to bilirubin (r = -0.56). No significant association was found between protein C and aspartate aminotransferase, alaline aminotransferase, and gamma-glutamyltranspeptidase. In conclusion, protein C is low in advanced liver diseases and gives the same amount and type of information as Normotest, antithrombin and heparin cofactor II.