Previous studies of sister-chromatid exchange (SCE) in patients with hepatitis B have been reported. But as far as we know, no such work has been done in children born to parents with hepatitis B, either one or both of whom are infected. In the present study, frequencies of SCE in the peripheral lymphocytes of 30 hepatitis B parents with hepatitis B surface antigen (HBsAg) positive and 40 of their children were observed. SCE frequencies of 20 normal adults and 3 normal children were analysed for comparison with the patients and their children. The results obtained from all of the samples were as follows: The hepatitis B patients with HBsAg positive had a significantly higher SCE frequency than the normal adults (P less than 0.01); the children born after their parents contracted hepatitis B had a significantly higher SCE frequency than normal children (P less than 0.01); there was no significant difference in SCE (P greater than 0.05) between children born after their parents contracted hepatitis B, children born after their mothers acquired it and children born after their fathers acquired it. The above results indicate that hepatitis B patients with HBsAg positive and their children born after they contracted hepatitis B had significantly higher frequencies of SCE; these data might throw new light on the study of genetic factors acting on the mechanism of hepatitis B.