An acute lymphocytic leukemia patient underwent allogeneic bone marrow transplantation (BMT) from a sibling who was serologically positive for syphilis. After the donor was administered antibiotic therapy, the titration of treponema pallidum hemagglutination (TPHA) decreased from x1260 to x320. Thereafter, the graft consisting of mononuclear cells was transplanted. TPHA of the recipient turned positive on day +63, but became negative 1.5 years after BMT. Although the cause of the seroconversion of TPHA seemed to be the contamination of treponema to the graft, the adoptive transfer could not be ruled out as an another possible cause.