The aim of this study was to determine serum immunoglobulins, IgG subclasses, lymphocyte subsets, and serum protective antitoxin levels of tetanus and diphtheria, and to investigate specific antibody response to tetanus and diphtheria vaccines in children with cancer who have been treated for leukemias and solid tumors. Forty patients with different types of childhood malignancies were enrolled in this study and their lymphocyte subsets, serum Ig A, M, G and IgG subclass concentrations were determined at completion of chemotherapy and 6 months later. We measured serum diphtheria (D) and tetanus (T) antitoxin levels and investigated specific antibody responses against DT vaccines at 6 months. Only the leukemic children had low CD19+ cells at completion of chemotherapy and 6 months later. The patients with solid tumors had reduced CD4+ cells, but increased natural killer cells at completion of chemotherapy. Serum IgA and IgM levels were decreased in leukemic patients after chemotherapy. There were no IgG subclass deficiency. Forty-two per cent of the patients did not have protective serum T antitoxins. All patients produced high levels of DT antibodies by vaccination. Immune system changes recover by 6 months after cancer therapy in children. Children with solid tumors, as well as leukemias, should be followed-up in terms of immune deficiencies. A repeat dose of tetanus toxoid should be recommended at 6 months.