To examine CSF hydrodynamics, we studied 16 patients with idiopathic intracranial hypertension and 45 control subjects with a constant-pressure infusion method. Fifteen patients had 155 examinations up to 15 years after the onset of disease. In most patients, the disturbances of CSF hydrodynamics persisted for many years. We identified at least two mechanisms for the development of the increased CSF pressure: a rise of sagittal sinus pressure, probably explained by extracellular edema causing partial compression of the major venous sinus (type 1), or a low conductance with a compensatory increase in CSF pressure in order to sustain the bulk flow (type 2).