The effects of Hantaan virus (HTNV) on human endothelial cells (HECs) were investigated both in vivo and in vitro. The 76-118 strain or SR-11 strain of HTNV were inoculated into HECs monolayers respectively, and the virus antigens could be detected on the seventh day of the first passage after inoculation by immunofluorescent technique. The HTNV could also be isolated through cultures of Vero E-6 cells. HTNV particles and inclusion bodies together with various changes in the organelles were observed in the infected cells by transmission electronic microscopy (TEM), and the immunoenzyme positive virus particles were seen by immunoelectronic microscopy. Samples of skin biopsy were individually obtained from 14 cases of hemorrhagic fever with renal syndrome on the second to the fifth day after the onset of the illness. It was found that HTNV antigens were widely distributed in the cytoplasm of endothelial cells of the samples from 5 out of 14 cases by avidin-biotin-peroxidase complex staining, and morphologic changes of the endothelial cells similar to those observed in vitro, were also seen by TEM. The results indicated that HEC is one of the target cells susceptible to HTNV. The virus could invade and propagate in HECs, and could induce damage to the latter.