Serious vascular leakage is central to the pathogenesis of hantavirus infections. However, there is no evidence suggesting the hantavirus infection of endothelial cells directly causes obvious cell damage or morphological alteration either in vivo or in vitro. In this study, we examined whether Hantaan virus (HTNV) infection modifies the barrier function of endothelial cell monolayers upon the exposure to pro-inflammatory cytokines. Low levels (1 ng/ml) of tumor necrosis factor-alpha initially increased the permeability in both HTNV-infected and uninfected monolayers similarly. Thereafter, however, these monolayers showed significant difference. The HTNV-infected monolayers remained irreversibly hyper-permeable during the experimental period up to 4 days, while the uninfected monolayers completely recovered the barrier function. The prolonged hyper-permeability of HTNV-infected monolayers was not associated with cell death or gap formation in the monolayers, and was independent from their nitric oxide or prostaglandin production. These results are the first evidence that hantavirus infection modifies barrier function of endothelial cell monolayers and suggest that HTNV-infection of endothelial cells may contribute to the increased vascular leakage through the prolonged response to cytokines.