From a previous study of achondroplasia as well as from the observation of patients with hydrocephalus associated with craniostenosis, the authors have concluded that an increased superior sagittal sinus venous pressure (SSVP) could be the cause of the enlarged ventricles. However, other workers have demonstrated that an increased SSVP could be the consequence of increased intracranial pressure (ICP). Therefore, the authors undertook a study to determine if there was a physiological test that could distinguish between rare instances of increased SSVP caused by structural and irreversible narrowing of the sinus and those caused by increased ICP. In 20 hydrocephalic infants and children, pressure was simultaneously measured in the lateral ventricle, the superior sagittal sinus, and the jugular vein. Stable baseline pressures were recorded, as well as the variations observed after the withdrawal of an amount of cerebrospinal fluid (CSF) sufficient to lower ICP to zero. Similar recordings were taken after reinjection of an equal quantity of CSF. In all of the patients, SSVP was increased, but not as much as the ICP. In the cases of hydrocephalus without any associated cranial malformation, and therefore without any likely anatomical interruption of the sinus, CSF withdrawal induced a simultaneous decrease of ICP and SSVP. However, whereas ICP could be lowered to zero, SSVP never fell below the jugular venous pressure, which remained stable (around 5 mm Hg) throughout the recording session. Results were different when sinography demonstrated an anatomical interruption of the sinus, as in cases of hydrocephalus associated with achondroplasia or craniostenosis. In these cases, although ICP was normally lowered by CSF withdrawal, SSVP remained nearly unchanged, usually greater than the jugular venous pressure. The present study demonstrated that SSVP recording during ICP variations induced by CSF withdrawal permits differentiation between a reversible collapse of the sigmoid sinus due to increased ICP and a fixed obstructive lesion of the sinuses. Based upon this test and the results of sinography, the authors inserted a venous bypass between the lateral sinus and a jugular vein in three patients.