Acute encephalopathy accompanying influenza virus infection results in brain and systemic organ failure mainly through vasogenic edema with high levels of inflammatory cytokines, such as blood tumor necrosis factor (TNF)-a and interleukin (IL)-6, as well as the cytochrome c apoptosis marker. A highly virulent strain of avian influenza virus causes fatal infection in chickens by infecting vascular endothelial cells in systemic organs, inducing apoptosis therein. To verify the possibility of apoptosis induction by human influenza virus in infected human vascular endothelial cells, purified influenza virus-infected human umbilical vein endothelial cells (HUVECs) were examined using a tissue culture method. When pre-treated with TNF-a, influenza virus (Philippine strain, H3N2) promoted TNF-a induced apoptosis of HUVECs. Viral replication was confirmed in HUVECsinfected with the Philippine strain in the absence of TNF-a by measurement of the amount of infective virus in the culture supernatant using the tissue culture infectious dose (TCID) method, immunohistochemistry and real-time PCR. The number of influenza virus genomes in the infected HUVECs at 24 hr postinfection increased about fivefold compared to that just after virus adsorption. Many TUNELpositive influenza virus-infected HUVECs were observed using the TUNEL method. Furthermore, cleaved caspase 3 was also detected in influenza virus-infected cells by immunofluorescence staining. These results demonstrated that human influenza virus can infect and replicate in human vascular endothelial cells and induce apoptosis therein. J. Med. Virol. 80:1072- 1078, 2008. © 2008 Wiley-Liss, Inc.