The isolation and enlargement of the fourth ventricle after a ventriculoperitoneal (V-P) shunt was classified as "isolated fourth ventricle (IFV)". The term, "disproportionately large communicating fourth ventricle (DFV)" was first introduced by Scotti et al as being an enlarged fourth ventricle communicating with the third ventricle. The authors present a case of DFV after the resection of an astrocytoma. Upon recurrence of the tumor a second resection was carried out 5 years later. It was found that IFV had evolved because a cyst in the right temporal lobe was obstructing the aqueduct. After shunting of the tumor cyst, the aqueduct was again found to be patent and the fourth ventricle gradually decreased in size. A 34-year-old female presented headache, nausea, and a mild left hemiparesis. An initial CT scan demonstrated a fourth ventricle of approximately normal size and a right temporal mass. The first craniotomy revealed an astrocytoma. A CT scan after the surgical procedure showed enlargement of all ventricles, especially the fourth, resulting from the blockage of the foramina of Luschka and Magendie. The insertion of a V-P shunt was followed by a reduction in size of all ventricles. The diagnosis of DFV was thus confirmed because the fourth ventricle had a demonstrated communication with the third ventricle. After a second craniotomy for tumor recurrence five years later, a CT scan revealed the enlargement of the fourth ventricle and a cyst in the right temporal lobe. A metrizamide CT scan revealed that the cyst was isolated and an RI ventriculogram confirmed obstruction of the aqueduct.(ABSTRACT TRUNCATED AT 250 WORDS)