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A previously well 17 year old female presented with a self-limiting episode of derealisation and depersonalisation lasting 24 hours. Initial MRI demonstrated new left peri-trigonal white matter hyperintensity. Initial bloods were normal and serum was negative for white cell enzymes, VLCFA and broad autoimmune screen but positive for GABA-b antibodies (Ab).Six months later, an acute cognitive decline, intermittent upper limb tremor and emotional lability led to readmission. On examination she had an ACE-R of 88/100 and mildly slowed saccades. Repeat MRI was stable and EEG was diffusely slow, particularly the right temporal lobe. She had a mild CSF lymphocytosis (8 cells). An underlying malignancy was excluded with CT chest, abdomen, pelvis and PET CT. She was treated with PLEX and pulsed intravenous methylprednisolone, which led to an improvement of her symptoms and cognitive status. She was discharged on oral steroid taper. One month later she had a milder relapse and was retreated with PLEX and started on mycophenolate. She has now been in remission for 18 months.Our case highlights that neurologists should consider GABA-b Ab mediated encephalitis as a differential in patients presenting with derealisation and depersonalisation, in addition to previously described presentations of seizures and ataxia.