The CSF flows in the aqueduct and at the foramen magnum were examined in 5 patients with communicating hydrocephalus (HC) and in 10 with benign intracranial hypertension (BIH) as well as in 5 healthy volunteers. As compared to normal individuals, the aqueductal flow in HC was about 10 times larger and the cervical flow was half as large. In BIH the CSF flows were not different from those of normal volunteers. The decreased arterial expansion as reflected in the reduced cervical flow in HC may be due to pathologic changes in the arteries and paravascular spaces. The large aqueductal flow in HC reflects a large brain expansion, causing increased transcerebral mantle pressure gradient and ventricular dilatation. In BIH there is a normal brain expansion (aqueductal flow) and consequently no ventricular dilatation. It is argued that BIH be caused by an obstruction on the venous side, as opposed to the vascular alterations in HC, which are on the arterial side.