Mucus-secreting adenomas or acinar adenoma of the choroid plexus are very rare. We report the case of a 79-year-old male with a 3-year history of occipital headaches with vomiting, ataxia and cerebellar signs. He was first seen due to difficulty while walking. He was admitted to the hospital with significant tumor expansion and clinical deterioration. CT and MRI revealed obstructive hydrocephalus secondary to a large fourth ventricular cyst mass, which enhanced markedly on contrast administration. Pathological findings were consistent with an acinar choroid plexus adenoma. The tumor was attached to the ependymal lining and was strongly adhered to the walls and floor of the IV ventricle. Post-operative bleeding complicated partial removal of this tumor. The patient died 6 h after surgery.