Brain tumors in the neonatal period most commonly appear as hydrocephalus without substantial localizing signs. Ventriculography and pneumoencephalography can detect the presence of a mass lesion even at this age. A neonate had a mass lesion that was shown by air ventriculography to be obstructing the aqueduct of Sylvius. This proved to be a polymorphic sarcoma. This case represents, to our knowledge, the second such congenital tumor to be reported.