For non-communicating hydrocephalus, neuroendoscopic third ventriculostomy has become a major choice. But sometimes, the procedure results in failure. Typically, impairment of a distal CSF absorption, a preexisting arachnoid membrane just below the fenestrated site and a glial scarring of fenestrated site were pointed out as a factors of failure. On the other side, the intraventricular pressure dynamics of a functioning third ventriculostomy is in the process of study. Recently some reports have noticed the importance of the flow of CSF into the prepontine cistern, mimicking the flow through the aqueduct of Sylvius. We report an unsuccessful trial of third ventriculostomy in a case with huge posterior fossa tumor.