Serologic responses to hepatitis B vaccine were investigated in 40 pediatric cancer patients, 20 with hematological malignancy and 20 with various solid tumors. in both of the groups ages of the patients were under 15 years and 14 patients were receiving chemotherapy and 6 were not receiving chemotherapy. All children were seronegative for hepatitis B virüs and had normal hepatic functiön. They received 40 $mu$g of recombinant hepatitis B vaccine by injection into the deltoid muscle at 0,1 and 2 months and quantitative antiHBs concentrations were determined at 1,2,3 and 7th months. A titer equal to ör greater than 10 IU/L was considered seropositive. The seroconversion rate in patients receiving chemotherapy after 3 vaccine doses was 0% in the hematological malignancy group and was 23.1% in the solid tümör group. in patients not receiving chemotherapy these rates 83.3% in both groups. The serologic response was found better in patients with solid tumors (p>0.05) and in patients not receiving chemotherapy (p<0.05).