This report describes the use of the recent three-dimensional Fourier transform constructing imaging in the steady state (CISS) MR sequence in the management of obstructive hydrocephalus. It is a gradient-echo imaging technique with high resolution which remains sensitive to flow. It enables locating the obstruction and determining the upstream impact. It provides anatomical information about third ventricle (V3) morphology and relationships useful before ventriculostomy. Twenty patients with obstructive hydrocephalus at the level of cerebral aqueduct or posterior V3 underwent sagittal 3DFT-CISS acquisition, supplemented by frontal, axial and coronal reformations in the cerebral aqueduct axis. 9 patients were examined after ventriculostomy with the same protocol. CISS-3DFT allows good visualization of the cerebral aqueduct and diagnosis of the underlying cause of obstruction (malformation, tumor), sometimes better than classical sequences. In case of complete obstruction, the flow-related signal void is usually absent in the cerebral aqueduct or V3. The position of V3 floor and its relationship with the tip of basilar artery are well analyzed. The dilatation of the anterior V3 recesses is a better sign of activity of hydrocephalus - and then of therapeutical indication - than classical transependymal resorption signs which are not always present in chronic longstanding adult hydrocephalus. Of the 9 ventriculostomy patients seven had linear flow-related signal void through V3 floor, from anterior V3 to basal cisterns on the postoperative MR study. This flow void confirms patent ventriculostomy.