Endoscopic third ventriculostomy(ETV)is the first-line treatment for fourth ventricle outlet obstruction(FVOO)-associated hydrocephalus. However, because FVOO is difficult to diagnose in the acute stage, ventriculoperitoneal shunt(VPS)is also used. Herein, we report two cases of shunted FVOO resulting in overdrainage or slit ventricle syndrome(SVS)that were treated successfully with the shunt-clamp system. In addition, we discuss the efficacy of the shunt-clamp system for FVOO-associated hydrocephalus. CASE 1:A 79-year-old man complained of severe postural headaches. One year earlier, he underwent VPS for secondary hydrocephalus associated with hemorrhagic cerebellar infarction. CT revealed that the ventricle had become slit-like. Although the shunt valve adjusted the maximum pressure, his complaint and the ventricle shape did not improve. After the on-off valve was inserted in the shunt system and clamped, his symptoms were resolved and the ventricle size was normalized. CASE 2:A 21-year-old man who complained of drowsiness, diplopia, and severe intermittent retroocular pain was admitted to our hospital. One year earlier, he underwent VPS with the shunt-clamp system for a secondary hydrocephalus after surgery for medulloblastoma. CT on admission revealed ventricle dilatation;however, the shape of the ventricle became slit-like 3 days after admission. We made a diagnosis of SVS and planned ETV. Owing to the difficulty in approaching the lateral ventricle, the shunt system was clamped 8 hours before the operation. After confirming ventricle dilatation, ETV was successfully performed. After the operation, the symptoms were resolved, and magnetic resonance imaging confirmed that the ventricle size was normalized.