67-year-old woman with pmh including hypertension and hypothyroidism arrived by EMS for unresponsiveness. Family heard her call for help and found her on the floor. Family reported strange behavior for few days preceding this event. On arrival to ED, she was drowsy, confused and unable to recall the event. CT head was unremarkable. Her mental status fluctuated, and she subsequently had a generalized tonic-clonic seizure requiring intubation for airway protection. Continuous EEG demonstrated frequent electrographic seizures originating from the left temporo-occipital region, ultimately requiring 4 anti-epileptics. MRI brain showed left temporal non-enhancing T2-FLAIR hyperintensities with mild restricted diffusion. CSF was unremarkable including HSV PCR. After extubation two days later, she demonstrated memory loss and intermittent confusion. Repeat MRI brain one week later showed persistent T2-FLAIR hyperintensities. MR spectroscopy was unremarkable. Repeat CSF studies were negative including autoimmune encephalitis panel and 2ndHSV PCR. Empiric acyclovir was discontinued. Serum autoimmune encephalitis panel was negative. Given diagnostic uncertainty, brain biopsy was performed and showed edematous and vascular changes with preservation of normal parenchymal architecture and absence of tumor and inflammatory cells. A diagnosis of PRES was considered, and hypertension was more aggressively treated. Mental status normalized and repeat MRI 2 months later showed partial resolution of T2-FLAIR hyperintensities.