Serologic responses to hepatitis B vaccine were investigated in 197 pediatric cancer patients. The patients, ages 1 to 21 years, comprised 66 with solid tumors, 101 with hematologic malignancies and 30 with various benign conditions. Of them 51 were receiving cytotoxic chemotherapy and 114 had not received chemotherapy for 0.2 to 11 years. Three doses of plasma-derived hepatitis B vaccine (20 micrograms) were given at 0, 1 and 6 months; and antibody concentrations to hepatitis B surface antigen were determined at 3, 6 and 8 months. The geometric mean antibody concentration after 3 vaccine doses was 1076 mIU/ml in cancer patients receiving chemotherapy and 18,833 mIU/ml in cancer patients not receiving chemotherapy. The protective titer of antibody (> or = 10 mIU/ml) was reached after 3 doses of vaccine by 67% of patients receiving chemotherapy and by 97% of those not receiving chemotherapy. The patients being treated for solid tumors had weaker responses than those being treated for hematologic malignancies: after 3 vaccine doses no response was observed in 6 of 11 patients with solid tumors compared with 3 of 25 of patients with hematologic malignancies. Children receiving anticancer chemotherapy have essentially weaker responses to hepatitis B vaccine than children not receiving chemotherapy or those with benign conditions. This reflects the profound immunosuppression during chemotherapy. The effect of more intensive immunization schedules should be investigated.