Abstract Cancer patients are often encouraged to receive seasonal influenza vaccination. The monoclonal antibody rituximab is widely used in treatment of non-Hodgkin lymphoma. This results in a prolonged depletion of normal B cells, which might impair humoral responses. The aim of the present study was to investigate whether lymphoma patients undergoing rituximab-containing treatment regimens or having received such regimens within the past 6 months were able to mount protective antibody responses to the influenza A(H1N1) 2009 virus vaccine Pandemrix during the 2009 “swine flu” pandemic. Contrary to the control group, where 82% responded adequately to the vaccine, none of the 67 patients achieved protective antibody titers, suggesting that lymphoma patients receiving rituximab-containing regimens might not benefit from this vaccine. It is important that doctors who care for such patients are aware that they may fail to respond not only to the influenza vaccine, but also to other common vaccines.