Natural killer cell activity in patients with liver cirrhosis relative to severity of liver damage. To evaluate the role of severe liver damage on natural killer cell activity, 29 patients with liver cirrhosis were examined. The natural killer cell activity was measured with a 4-hr chromium release assay, and the K562 cell line was employed as target cells. The natural killer cell activity was significantly decreased in cirrhotic patients compared with normal controls and patients with chronic active hepatitis. Cirrhotic patients with Pugh's C grade of severity of liver disease had lower natural killer cell activity. The depression of natural killer cell activity in cirrhotic patients was inversely correlated with prothrombin time ratios, and the natural killer cell activity in cirrhotic patients with hepatic encephalopathy was lower than in patients without hepatic encephalopathy. Thus, the diminished natural killer cell activity in cirrhotic patients might be related to the severity of liver damage. 