A patient with pseudotumor cerebri and Guillain-Barré syndrome was studied with serial measurements of pressure-volume index, CSF outflow resistance (Ro), and CSF production rate. The results were comparable to findings in previous cases of idiopathic pseudotumor. Although Ro was elevated and progressively diminished as the pseudotumor syndrome improved, the extent of elevation in Ro was inadequate to account for raised CSF pressure. These results suggest an alternative explanation for pseudotumor based on raised effective venous pressures at points of CSF outflow that are passively reflected in raised CSF pressure. A parallel rise in vascular and CSF pressure may also explain why patients with pseudotumor tolerate such high intracranial pressures.