To evaluate the significance of natural killer (NK) cell activity in the clinical assessment of patients with hepatocellular carcinoma (HCC), 32 patients combined with liver cirrhosis (LC) and HCC, and 29 LC patients were studied. The NK cell activity was markedly decreased in HCC patients and the LC group as compared with the control group, but there was no statistical difference between the NK cell activity of the HCC group and the LC group. The depression of NK cell activity in HCC patients was inversely correlated with the patient's age, and the HCC patients with venous invasion or with both lobes involved had lower NK cell activity. These results suggest that the decreased NK cell activity in HCC patients might be related to the coexistent liver disease, and marked decrease in NK cell activity might be one of the causes for the early development and invasion of HCC.