When compared to 32 healthy normal weight normolipidemic control subjects, plasma protein C antigen and serum cholinesterase activity were significantly decreased in 17 patients with decompensated cirrhosis of the liver and in 29 critically-ill surgical patients displaying the acute phase reaction, most of them without evidence of consumption coagulopathy. The low levels of these variables are considered to be subsequent to impaired and dysregulated hepatic protein synthesis. On the contrary, plasma protein C and serum cholinesterase were increased in 20 nephrotic patients and in 20 overweight hypertriglyceridemic subjects, a finding highly suggestive of enhanced hepatic synthesis probably related to an accelerated turnover of triglycerides. A discrepancy between low serum cholinesterase activity and normal or even high plasma protein C antigen was noted in 15 patients with cholestasis. This was particularly evident in 7 subjects with extrahepatic cholestasis and an abnormal pattern of hepatic protein synthesis or impaired clearance of plasma protein C would appear to develop in such pathological conditions.