To determine the frequency of hepatitis B virus (HBV) transmission by hepatitis B surface antigen (HBsAg) carriers in school classes 44 highly viremic children were examined over an average period of 5 years. All these HBsAg carriers had been infected with the virus in a hospital endemia during treatment for childhood malignancies. They have been continuously positive for HBeAg with very high HBV-DNA titers of more than 10(8) viral genomes/ml serum. Due to data protection reasons no serologic tests could be done in these school classes. Therefore, the patients and their parents were asked to fill in a questionnaire about any cases of hepatitis in the school classes during the study period. These results were compared with the hepatitis B registrations in the records of the public health offices. No differences were found between the two sources of data. In the school classes of the HBsAg carriers the incidence of hepatitis cases was 0.3 per 1000 pupils per year, which is not different from the hepatitis B morbidity within the general population. Therefore, there is no reason to impose any restrictions on HBsAg carriers in school. In contrast, several cases of HBV transmissions occurred to non-vaccinated family members of the HBV carriers.