Adult T-cell leukemia (ATL) occurs in a small population of human T-cell leukemia virus type 1 (HTLV-1)-infected individuals. Although the critical risk factor for ATL development is not clear, it has been noted that ATL is incidentally associated with mother-to-child infection, elevated proviral loads, and weakness in HTLV-1-specific T-cell immune responses. In the present study, using a rat system, we investigated the relationships among the following conditions: primary HTLV-1 infection, a persistent HTLV-1 load, and host HTLV-1-specific immunity. We found that the persistent HTLV-1 load in orally infected rats was significantly greater than that in intraperitoneally infected rats. Even after inoculation with only 50 infected cells, a persistent viral load built up to considerable levels in some orally infected rats but not in intraperitoneally infected rats. In contrast, HTLV-1-specific cellular immune responses were markedly impaired in orally infected rats. As a result, a persistent viral load was inversely correlated with levels of virus-specific T-cell responses in these rats. Otherwise very weak HTLV-1-specific cellular immune responses in orally infected rats were markedly augmented after subcutaneous reimmunization with infected syngeneic rat cells. These findings suggest that HTLV-1-specific immune unresponsiveness associated with oral HTLV-1 infection may be a potential risk factor for development of ATL, allowing expansion of the infected cell reservoir in vivo, but could be overcome with immunological strategies.