The response to hepatitis B (HBV) vaccination was studied in 32 breast cancer patients who were HBV antigen- and antibody-negative at the time of diagnosis. Ten of the patients included into the study received radical mastectomy without adjuvant therapy, 15 patients underwent breast-conserving surgery with subsequent irradiation with or without tamoxifen treatment, and 7 patients were treated by modified radical mastectomy plus adjuvant chemotherapy. Eight patients received the first dose of vaccine 3 months after irradiation therapy. Seven patients were first vaccinated 6 months following irradiation therapy. Seven healthy age-matched persons were vaccinated as controls. Antibody response in the patients was significantly delayed. Four weeks after the second vaccination 6 out of 7 controls showed a significant (> 10 IU/ml) HBs antibody titer, while only 6 out of 32 patients responded. In addition, HBs antibody titers were also significantly lower after the second vaccination even in those patients treated with surgery alone, although they were first vaccinated 3 months after surgery and had no residual tumors. This difference disappeared after the second booster immunization, at which time the frequency of significant antibody titers and the levels of HBs antibody titers were comparable between patients and controls. As the population of patients undergoing treatment for breast cancer is increasing, additional studies are needed to determine optimal immunization regimens in this group of patients.