Avian influenza A (H9N2) virus is transmitted sporadically from avian species to human causing mild diseases in immunocompetent person. We report two immunocompromised patients cases of human infection by the virus in Hong Kong between 2008 and 2009. One patient had uneventful recovery with viral shedding at day 10 after symptom onset despite her underlying acute lymphoblastic leukaemia. The other patient with post-bone marrow transplant chronic graft versus host disease and bronhioltis obliterans went into respiratory failure. Genetic analysis revealed that these cases were caused by different genetic variants which are circulating in poultry in this region. Review of literature identified another 9 human cases reported in Southern China since 1988. It is possible that human infection with H9N2 is more common than what has been recognized. Continuous surveillance of H9N2 influenza virus infection in human is warranted