A 50-year-old woman developed a human herpes virus 6 limbic encephalitis characterized by memory loss and somnolence, 3 weeks after a cord blood-derived stem cell transplantation. Sequential scalp electroencephalogram failed to detect seizure activity. Cerebrospinal fluid polymerase chain reaction assay demonstrated human herpes virus 6 deoxyribonucleic acid (positive, 3.74 log). Acute phase FDG PET imaging showed bilateral intense FDG uptake in both hippocampi and amygdalae. After antiviral treatment (gancyclovir followed by foscarnet), neurologic symptoms disappear gradually. Late phase FDG PET imaging showed a regression in FDG uptake representing hippocampal hypometabolism because of hippocampal sclerosis.