The effects of the human T-cell leukemia virus type I (HTLV-I) on cultured human endothelial cells were evaluated. Coculture of endothelial monolayers with either irradiated HTLV-producing lymphocytes or cell-free virus resulted in the production of multinucleated syncytia. The development of syncytia was inhibited by sera from patients with adult T-cell leukemia/lymphoma (ATLL). HTLV antigens were present on endothelial syncytia passaged in culture for greater than 3 months as detected by an anti-p19 monoclonal antibody, which detects a core protein of HTLV-I, and by ATLL sera. Moreover, these HTLV-infected endothelial cells were then able to infect and transform normal cord blood T lymphocytes with HTLV. These studies demonstrate that human endothelial cells are susceptible to productive HTLV-I infection in vitro and may have relevance for the spectrum of human disease associated with this family of retroviruses.