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There is little known about the impact of the timing of influenza vaccine administration on seroconversion in patients on chemotherapy. Recommendations for other vaccines state that the vaccines should be readministered several months after the completion of chemotherapy outside of the stem cell transplant setting. This is not often possible with the influenza vaccine because of its seasonal nature. To examine whether certain times during chemotherapy are more favorable for seroconversion, we examined vaccine responses in a cohort of children on chemotherapy. Pediatric patients on chemotherapy were recruited over the 2006 to 2008 influenza vaccine seasons. Sixty-eight acute lymphoblastic leukemia (ALL), 3 acute myeloid leukemia, and 18 sarcoma patients were evaluated. Clinical and laboratory features were recorded. The hemagglutination inhibition (HAI) assay was used to define serotype-specific responses. Seroconversion rates varied according to the type of chemotherapy during the vaccination period. In some cases, there was a late rise in titer, suggesting that a wild-type infection had occurred, leading to an estimate of vulnerability of this population. In patients with ALL, responses to the vaccine were greater when it was given early in the course of treatment. We conclude that seroconversion rates are well below the rates cited for the general population. The 3 acute myeloid leukemia patients had a particularly poor response to the vaccine. In the case of ALL patients, it may be possible to adjust the timing of the vaccine to optimize the response.