Malignancy and its treatment are major causes of secondary immunodeficiency in childhood. The authors investigated the effects of chemotherapy on humoral immunity against hepatitis B, tetanus, and diphtheria in children with hematologic malignancies. The authors recruited 54 patients with hematologic malignancies after the completion of chemotherapy (group A), 25 patients with newly diagnosed hematologic malignancies before initiation of chemotherapy (group B), and 74 healthy controls (group C). All participants had been vaccinated against hepatitis B, tetanus, and diphtheria according to the Iranian national vaccination scheme. Patients in group A achieved protective levels of diphtheria and hepatitis B antibodies significantly less frequently than the other 2 groups and protective levels of tetanus antibody significantly less frequently than group C (P <.05). After controlling for age, the association observed for tetanus lost its significance, but chemotherapy was a significant and independent predictor of failure to achieve protective levels of antibodies against diphtheria (odds ratio [OR] = 7.7, P < .001) and hepatitis B (OR = 3.13, P = .008). These results indicate that chemotherapy has independent adverse effects on vaccine-induced antibody protection against diphtheria and hepatitis B.