The rate of micronucleus formation in lymphocytes was determined in 42 patients (including 10 acute icteric hepatitis B, 15 chronic active hepatitis B (CAH), 8 liver cirrhosis and 9 liver cancer) and 13 normal subjects. The results showed that the rate of micronucleus formation in lymphocytes in the patients with CAH (12.267 +/- 5.298%), liver cirrhosis (12.375 +/- 8.551%) or liver cancer (19.444 +/- 13.324%) was markedly higher than that in those with acute icteric hepatitis B (5.400 +/- 1.430%) or normal subjects (3.308 +/- 1.284%) (P less than 0.01). The rate of micronucleus formation in lymphocytes is higher in the liver cancer group than that in the CAH group or cirrhosis group (P less than 0.05). The rate of presence of two or more micronuclei in the lymphocytes was obviously higher in the liver cancer group (3.667 +/- 4.743%) than that in the liver cirrhosis group (1.500 +/- 1.690%), CAH group (1.467 +/- 1.807%), acute icteric hepatitis B group (0.600 +/- 1.075%) or healthy group (0.462 +/- 0.660%) (P less than 0.01 or less than 0.05). This method is much simpler than the measurement of chromosomal damage, and its reliability is as good as the latter. Measurement of micronuclei in lymphocytes can reflect the degree of liver damage in patients with the infection of hepatitis B virus. It may be used as the subclinical marker of the patients with liver cancer too.