Protein Z is a vitamin K-dependent plasma protein, which promotes the association of thrombin with phospholipid surfaces. Deficiency states of protein Z have recently been showr in patients with a bleeding tendency of an otherwise unknowr origin. In the present study severe diminutions of blood plasma protein Z were found in patients with chronic liver diseases. Protein Z levels decreased with increasing severity of liver disease. Strong correlations between protein Z and other plasma proteins of liver origin indicate that protein Z primarily originates from the liver. The cutaneous bleeding tendency in cirrhosis may in part be due to protein Z deficiency.