The authors have routinely inserted a ventriculoperitoneal shunt in all patients with a posterior fossa tumor and hydrocephalus some 7-10 days prior to craniotomy. Forty-one patients with medulloblastoma were treated in this fashion and of these, four metastasized through the shunt and died of systemic metastases without evidence of recurrent tumor in their central nervous system. A millipore filter which can be incorporated in the shunt has been used by the authors during the past 18 months in an effort to prevent metastatic spread through the shunt.