[en] Hematopoietic stem cell transplantation is the only curative treatment for several hematological malignancies and immune deficiency syndromes. Nevertheless, the development of graft-versus-host disease (GvHD) after transplantation is a severe complication with high morbidity and mortality. The aim of this study was to image human T cells during GvHD development and their migration into GvHD-related organs. By using a radiolabeled anti-human CD${}_3$ monoclonal antibody (mAb), we were able to visualize GvHD progression in a humanized mouse model. Human peripheral blood mononuclear cells (PBMC) were transferred into immunodeficient mice (initially n = 11 mice/group) to induce GvHD. One group additionally received regulatory T cells (Treg) for prevention of GvHD. T cell migration was visualized by sequential small animal PET/MRI using ${}^{89}$Zr-labeled anti-human CD${}_3$ mAb. Flow cytometry and immunohistochemistry were used to measure T cell frequencies in relevant organs at different time points after engraftment. Using radiolabeled anti-CD${}_3$ mAb, we successfully visualized human T cells in inflamed organs of mice by ${}^{89}$Zr-anti-CD${}_3$-PET/MRI. Upon GvHD progression, we observed increased numbers of CD${}_{3^{+}}$ T cells in the liver (22.9% on day 3; 94.2% on day 10) and the spleen (4.4% on day 3; 58.8% on day 10) which correlated with clinical symptoms. The liver showed distinct spot-like lesions representing a strong focal accumulation of T cells. Administration of Treg prior GvHD induction reduced T cell accumulation in the liver from 857 ± 177 CD${}_3$${}^{+}$ cells/mm${}^2$ to 261 ± 82 CD${}_3$${}^{+}$ cells/mm${}^2$ and thus prevented GvHD. ${}^{89}$Zr-labeled anti-human CD${}_3$ mAb can be used as a proof of concept to detect the exact spatio-temporal distribution of GvHD-mediating T cells. In the future, radiolabeled T cell-specific mAb could be employed as a predictive early biomarker during the course of GvHD maybe even before clinical signs of the disease become evident. Furthermore, monitoring T cell migration and proliferation might improve tailored GvHD therapy.