Choroid plexus cysts are common and typically asymptomatic abnormal folds of the epithelial lining of the choroid plexus. Rarely, these cysts may gradually enlarge and cause outflow obstruction of cerebrospinal fluid. The authors present a case of a large choroid plexus cyst causing acute hydrocephalus in a previously healthy 2-year-old boy. The patient presented with markedly declining mental status, vomiting, and bradycardia over the course of several hours. Computed tomography scans demonstrated enlarged lateral and third ventricles with sulcal effacement, but no obvious mass lesions or hemorrhage. There was no antecedent illness or trauma. A right frontal external ventricular drain was placed in the patient, resulting in decompression of only the right lateral ventricle. Magnetic resonance (MR) imaging demonstrated a lobulated cyst arising from the choroid plexus of the left lateral ventricle and herniating through the foramen of Monro into the third ventricle, occluding both the foramen of Monro and the cerebral aqueduct. The patient underwent an endoscopic fenestration of the cyst, and histological results confirmed that it was a choroid plexus cyst. Postoperative MR imaging showed a marked reduction in the cyst size. The cyst was no longer in the third ventricle, the foramen of Monro and the aqueduct were patent, and the ventricles were decompressed. The patient was discharged home with no deficits. To the authors' knowledge, there are no previous reports of a choroid plexus cyst causing acute hydrocephalus due to herniation into the third ventricle. This case is illustrative because it describes this entity for the first time, and more importantly highlights the need to obtain a diagnosis when a patient presents with acute hydrocephalus without a clear cause.