The occurrence of hydrocephalus in association with a vestibular schwannoma (VS) is a well-known phenomenon. It is usually supported, albeit never demonstrated, that radiosurgery increases the risk of hydrocephalus. The purpose of this study is to investigate this Issue in our own series of patients in order to provide more data. Between July 1992 and January 2002, among the 1000 VS that have been treated at the Timone hospital using a Gamma knife, 43 patients displayed a hydrocephalus, 32 of them before the treatment (group A) and 11 of them only after the treatment (group B). It is of note that in both groups, age at the time of treatment (median age of 70 in A & B) and Volume of tumor were comparable and significantly higher than for the whole treated population. Following radiosurgery, 75% of the patients from the group A did not require a shunt whereas all the patients from the group B did, including 3 who also had significant tumor progression requiring surgery. Occurrence of a de novo hydrocephalus was a rare event (1%) that required a shunt early after radiosurgery, at a mean interval of 14.8 Months (4-31). Results from this study suggest that radiosurgery does not significantly increase the risk of hydrocephalus during the natural history of a VS. We can postulate that Gamma Knife radiosurgery might provide a protective influence on hydrocephalus decompensation since the number of preexisting hydrocephalus that necessitated a shunt after the treatment was small. More investigations involving more patients will bring more arguments in the near future.