A case of obstructive hydrocephalus and bilateral cerebellitis with swelling in a 14-year-old boy is reported from the Department of Child Health, St George’s Hospital, and the Departments of Neurosurgery and Neuroradiology, Atkinson Morley’s Hospital, London, UK. The child presented with a 2-month history of recurrent headache and a 2-week history of occipital headache, nausea and vomiting, without signs of acute infection or immunization. Fundi showed bilateral papilledema. Apart from mild dysdiadochokinesia, the neurologic exam was otherwise normal. MRI showed obstructive hydrocephalus and marked bilateral cerebellar swelling with increased signal on T-2 weighted imaging, compatible with acute inflammation. Clinical and MRI signs resolved in 4 weeks following treatment with corticosteroids, and the neurologic exam was normal at 12 month follow-up. 
COMMENT. The clinical and MRI findings and recovery following steroids were considered consistent with a parainflammatory cerebellitis, although viral studies were negative. Prompt diagnosis and therapy are important because of the danger of tonsillar herniation.