HTLV-1 is an important factor involved in various diseases including adult T-cell leukemia/lymphoma and HTLV-1 associated myelopathy/tropical spastic paraparesis. Amount of HTLV-1 provirus integrated in human peripheral blood mononuclear cells might be a candidate for a risk factor in the manifestation of HTLV-1 associated diseases. Experimental animal models would be useful to dissect the pathogenesis of HTLV-1 associated diseases. We present rat and mouse models of HTLV-1 infection. Using these animal models, we could clarify the intrauterine transmission of HTLV-1, and have found that both genetic background and HTLV-1 infection are important in pathogenesis of HAM/TSP-like rats. We also discuss the preventive measures of HTLV-1 transmission by use of antisense oligonucleotides.