The conditioning chemotherapies for hematopoietic stem cell transplantation (HSCT), particularly the powerful conditioning regimen that includes total body irradiation can result in serious infectious diseases due to the associated hypoglobulinemia. To counteract this effect, intravenous immunoglobulin (IVIG) therapy may be performed. However, IVIG therapy can also drive regulatory T-cell (Treg) production via stimulation by a T cell epitope (Tregitope) for major histocompatibility complex (MHC) Class II. In the present study, we measured a subset of Treg-related biomarkers, i.e., interleukin (IL)-10, IL-12, transforming growth factor (TGF)β1, and soluble cytotoxic T lymphocyte-associated antigen 4 (sCTLA-4) in HSCT patients and examined the relationship between these biomarkers and IVIG. Significant elevations of the IL-10, TGFβ1, and sCTLA-4 levels were observed in the IVIG group compared with the non-IVIG group, whereas the IL-12 levels exhibited a decreasing trend in the IVIG group. These results suggest the possibility that IVIG could have a therapeutic effect for Treg-related complications such as graft-versus-host disease after HSCT.