Tumefactive multiple sclerosis refers to the presentation of large demyelinating lesions (≥2 cm in diameter) mimicking brain tumors clinically and radiologically. We present the MRI and FDG PET/CT findings in a case with tumefactive multiple sclerosis, who had chronic graft-versus-host disease after allogeneic hematopoietic stem cell transplantation. Head MRI showed 7 cerebral lesions with incomplete ring enhancement. All but one lesion had size more than 2 cm. All these demyelinating lesions showed increased uptake at the rims of the lesions with central hypometabolism. Stereotactic brain biopsy of the right frontal lesion revealed extensive macrophage and lymphocyte infiltration.