We report here the first case report of bone marrow transplantation (BMT)-related transmission of human T lymphotropic virus type-I (HTLV-I). Antibodies against HTLV-I-associated antigens (anti-HTLV-I) were detected in the serum from the BMT recipient 12 days post BMT. IgG against gag core proteins (anti-p19 and anti-p24) appeared earlier than IgM against gag and env proteins (anti-p19, anti-p24 and anti-gp46) during seroconversion. The data presented here differs from blood transfusion-related seroconversion. This phenomenon may be due to the engraftment of anti-HTLV-I producing cells from the donor.