A 22-month-old boy presented with nausea and gradual deterioration of gait disturbance. Computed tomography (CT) demonstrated an intraventricular mass lesion in the right lateral ventricle. He was referred to our department 3 weeks after onset. Acute hydrocephalus gradually proceeded 4 days after admission, and external ventricular drainage (EVD) was performed. EVD revealed cerebrospinal fluid (CSF) overproduction (800-1,500 mL/day) under constant pressure of 10 cm H2O above external auditory meatus. Magnetic resonance imaging showed a multi-lobular mass in the inferior horn of the right lateral ventricle. A choroid plexus tumor was suspected. The ratio of blood urea nitrogen:creatinine (BUN:Cre) remained between 30 and 40, and hemoglobin was between 14.0-17.0 mg/dL, suggesting marked dehydration. Serum sodium varied between 117 and 140 mmol/L, and serum potassium between 2.2 mmol/L and 6.9 mmol/L. The amount of EVD was unstable and fluid balance management was difficult. Hypotonic fluid with sodium chloride supplement was used to adjust the fluid and electrolyte imbalance. Surgical removal of the tumor was performed 6 days after EVD and tumor was grossly and totally removed. The high BUN:Cre ratio decreased to about 15 and hemoglobin recovered to 7.5-9.0 mg/dL after removal. Electrolytes returned to the normal range. Overproduction of CSF also markedly improved to < 300 mL/day. Histopathological examination diagnosed choroid plexus papilloma.We experienced a case of choroid plexus papilloma associated with fluid-electrolyte imbalance due to over-drainage after EVD, which could not be effectively controlled before tumor removal. Cautious fluid management and emergent surgical resection might be required to manage the overproduction of CSF and fluid-electrolyte imbalance.