Background: Patients receiving chemotherapy are at increased risk for influenza virus infection. Little is known about the preferred moment of vaccination during chemotherapy. Patients and methods: Breast cancer patients received influenza vaccination during FEC (5-fluorouracil, epirubicin and cyclophosphamide)-containing chemotherapy regimens. Patients were randomised for early (day 4) or late (day 16) vaccination during the chemotherapy cycle. Influenza virus-specific antibody titres were determined before and 3 weeks after vaccination by haemagglutination inhibition. Results: We included 38 breast cancer patients (20 in the early and 18 in the late group) and 21 healthy controls. The overall patient group had significant lower responses to the vaccine compared with healthy controls. Patients vaccinated at day 4 tended to have higher antibody titres as compared with patients vaccinated at day 16, although the difference in post-vaccination titres is not statistically significant. Geometric mean titres post-vaccination for day 4 versus day 16 were 63.7 versus 29.5 (H3N2), 28.2 versus 19.6 (H1N1) and 29.8 versus 16.0 (B/Brisbane), respectively. Conclusions: Patients on chemotherapy have significantly lower responses to influenza virus vaccination compared with healthy controls. Vaccination early during the chemotherapy cycle induces better responses than does vaccination at day 16 of the cycle. Follow-up studies are needed to confirm this effect.