The immune response to influenza vaccination in children with cancer is controversial. The objective of this study was to characterize the cellular and humoral immune responses to an influenza vaccine in children with cancer who were receiving chemotherapy. In this study, children with cancer, who were not previously immunized, received an influenza vaccine via intramuscular injection. Blood samples were obtained prior to and at 4 weeks after immunization. Antibodies were measured using a hemagglutination inhibition (HI) assay. Cell-mediated immunity was measured by specific lymphoproliferation with (3)H-thymidine incorporation and by measuring cell frequencies following staining with monoclonal antibodies (CD8, CD4, CD19, CD45RA and CD27) using flow cytometry following incubation with the influenza antigen for 5 days. Geometric mean titers (GMT), mean counts per minute (cpm), cell frequencies prior to and following vaccination and percentage patient responses were compared using the Mann-Whitney non-parametric U and Chi-square tests; where p<0.05 was considered to indicate a statistically significant result. A total of 56 children were included. Their mean age was 6.64±3.61 years. Acute lymphoblastic leukemia (ALL) was diagnosed in 75, solid tumors in 23 and lymphoma in 2% of the children. Subjects with titers ≥40 hemagglutination units (HU) increased from 43% prior to vaccination to 73% following vaccination (p=0.01), whereas the GMT increased from 31.35 [95% confidence interval (CI), 29-111] to 143.45 HU (95% CI, 284-640) following vaccination (p<0.001). An increase in CD45RA expression in CD8(+) T cells was observed following vaccination (p=0.01). An increase in CD27 expression was observed in the CD4/8-negative cell population stimulated with the influenza antigen following vaccination (p<0.05). No serious adverse effects were observed. An increase in the seropositivity rate and GMT values following influenza vaccination were also observed. Influenza immunization was well tolerated among these children with cancer and increased the humoral and cellular immune responses with the activation of probable lymphoid precursors.