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A 52-year-old previously well man presented with 3-months of progressive intermittent headache, hearing loss and vertigo followed by 1-month of progressive ataxia and diplopia. Examination findings were consistent with a pan-cerebellar and brainstem syndrome with multidirectional nystagmus, square-wave jerks and cerebellar dysarthria.Investigations revealed a CSF lymphocytosis(15/mm3), elevated CSF protein(0.8g/L) and intrathecal oligoclonal band synthesis. CSF/serum surface and intracellular neuronal antibodies were negative. Whole-body PET/CT demonstrated no malignancy. Infectious, autoimmune and lymphoma investigations were negative.A testicular ultrasound showed small hypo-echoic foci with ill-defined margins. Biopsy demonstrated fibrosis with no malignancy.The patient continued to progress with increasing spasticity and clonus. He was treated for a probable paraneoplastic encephalitis: IV methylprednisolone, IVIG (total 2g/kg), plasma exchange and IV cyclophosphamide.KLHL11-antibody testing via a cell-based assay (Mayo Clinic) was positive. Subsequent bilateral orchidectomy revealed Bilateral Sertoli cell-only Syndrome with no malignancy. The patient remains clinically stable on monthly IV cyclophosphamide and oral prednisolone.KLHL11-antibody paraneoplastic syndrome is a rare cause of rhombencephalitis and are associated with testicular malignancy (germ cell tumours/teratomas). However, there are rare case-reports without malignancy and presentation prior to malignancy development. Clinicians should consider KLHL-11 antibody testing in patients with this distinct clinical picture despite negative malignancy screening.victorzhang93@gmail.com