We present a 2.5-year-old child suffering from acute hydrocephalus. First, the child was diagnosed with aseptic viral meningitis. The PCR of the cerebrospinal fluid (CSF) was positive for enterovirus. Subsequently, MRI revealed that the hydrocephalus was caused by a cyst in the third ventricle. During ventriculoscopy, the cyst had all aspects of an arachnoid cyst. An endoscopic fenestration and partial removal of the cyst was performed, combined with a ventriculocisternostomy. The coincidental finding of viral meningitis and a third ventricle arachnoid cyst in a patient with acute hydrocephalus has, to our knowledge, not been described in literature before. If there is a relation between the enteroviral meningitis, the arachnoid cyst (possibly causing a pre-existing subclinical hydrocephalus) and the rapidly evolving neurological deterioration, remains speculative. Proposed mechanisms, by which the viral meningitis could accelerate the disease process, are slight brain swelling or increased CSF production. This rare combination of diagnoses could also be coincidental.