Introduction:HHV-6 is a common infection that may resurface post-HSCT. The incidence of HHV-6 encephalitis is reported to be 0.96 to 11.6% after allogeneic HSCT. It is associated with a poor survival and neuropsychological sequelae in many cases. We report two cases of HHV-6 encephalitis treated with foscarnet. Case1. Allogeneic umbilical CBT after myeloablative conditioning (MAC) was performed in a 10-year-old girl with very high risk ALL, t(4;11) in 2nd CR. Neutrophil engraftment was achieved on D+13 after HSCT. On D+15, she had a tonic seizure and she showed lethargy, confusion, recent memory loss and ambulatory dysfunction. A brain MRI revealed high-signal lesions in bilateral hippocampi and temporoparietal cortex. CSF exam was negative for WBC,but multiplex PCR for HHV-6 was positive, and blood PCR was also positive. She was treated with foscarnet for 20 days. Her posttransplant course was complicated by acute GvHD, and severe GI bleeding necessitating ileocecal resection. Currently, 10 months after CBT she is in remission with complete chimerism. Her cognitive function has recovered gradually, but mild cognitive deficit and weakness still remain. Case2. A 17-year-old male with 47, XYY was diagnosed to have a mixed phenotype acute leukemia. An HSCT from matched unrelated donor after MAC was performed in CR1. Neutrophil recovery was observed on D+25. One day later atonic seizure followed by delirium, confusion, recent memory loss and disorientation developed. A brain MRI revealed abnormal high signal intensities in both hippocampi and diffuse leptomeningeal enhancement along hemispheres. CSF exam showed: WBC, 24/mm3; RBC, 12/mm3; protein 76.4 mg/dL and positive for HHV-6. He was treated with foscarnet for only 7 days due to acute kidney injury,but PCR for HHV-6 became negative both in CSF and blood. His course was complicated by bacterial sepsis, hemorrhagic cystitis, and pericarditis. Now 3 months after HSCT, he is in remission with chimerism. His mental status gradually improved,but he is still showing mild short-term memory loss and weakness. Conclusion:HHV-6 encephalitis is newly recognized, serious complication after HSCT. Characteristic amnesia after seizure early HSCT period should raise suspicion of possible HHV-6 limbic encephalitis. Prompt treatment with forscarnet should be initiated but neuropsychologic sequelae may remain permanently.