Description: BACKGROUND Cytopenia occurs frequently during cytotoxic chemotherapy. Little is known about the optimal timing of influenza vaccination for patients receiving chemotherapy. This study compared the immunogenicity of an influenza vaccine administered concurrently with chemotherapy (day 1) and within the cytopenic period (day 11) during 3-week cytotoxic chemotherapy cycles. METHODS Adult patients with solid cancer undergoing scheduled 3-week cytotoxic chemotherapy were randomly assigned to receive the 2014-2015 seasonal influenza vaccine on day 1 or 11 during the chemotherapy cycle. Patients were stratified by their age (〈60 and ≥60 years) and previous influenza vaccination status. Antibody responses to influenza vaccine strains H1N1, H3N2, and B were measured before and 21 to 28 days after vaccination with a hemagglutination inhibition antibody assay. RESULTS Ninety-seven patients were randomized into a day 1 group (n = 43) or a day 11 group (n = 54). Eighty-three patients were included in the final analysis. The mean age was 54 (± 11) years. Cancer types included breast (61%) and lung cancer (30%). Baseline characteristics were not significantly different between the groups. Seroprotection rates after vaccination were also not significantly different for the day 1 and 11 groups (strain H1N1, 67% vs 75% [ P  = .403]; strain H3N2, 77% vs 80% [ P  = .772]; strain B, 21% vs 27% [ P  = .472]). Seroconversion rates and postvaccination geometric mean titers were also similar for the groups. Vaccine-related adverse events were more common in the day 11 group (13% vs. 32%; P  = .040). CONCLUSIONS The antibody responses to influenza vaccination on days 1 and 11 during a 3-week cytotoxic chemotherapy cycle were comparable. Influenza vaccination can be performed concurrently with cytotoxic chemotherapy or during the cytopenic period. Cancer 2016. © 2016 American Cancer Society .