['T-cell responses have been implicated in control and exacerbation of lung injury during influenza A virus (IAV) infection.', 'To examine the breadth and magnitude of influenza-specific CD4(+) and CD8(+) T-cell responses during acute phase of infection.', 'Influenza-specific T-cell response to the entire pandemic H1N1/09 IAV proteome and T cell-related cytokine levels were measured in blood from previously healthy individuals with mild (n = 32) and severe (n = 16) IAV infection during the 2009 influenza pandemic. Virus-specific T-cell response in lung and blood was also performed in two acutely infected, severely ill patients using fluorescent-conjugated pdmH1N1/09 Matrix-MHC-I tetrameric complexes.', 'Strong and broad CD4(+) but not CD8(+) T-cell responses were observed in the blood, and were higher in those with severe disease. Antigen-specific CD8(+) T cells in the lungs were on average 45-fold higher compared with blood in severely ill patients. Paradoxically, in patients with severe disease, IL-17, IL-2, IL-4, and IFN-γ levels were significantly decreased.'] Do high levels of virus-specific CD4+ T cells predict severe pandemic influenza A virus infection?