Intracranial pressure, conductance to cerebrospinal fluid outflow, and cerebral blood flow were investigated in 14 patients with benign intracranial hypertension (pseudotumor cerebri). Intracranial pressure was increased in 9 patients (20 to 30 mm Hg), borderline in 4 patients (15 to 18 mm Hg), and normal in 1 patient (8 mm Hg). Six patients had plateau waves, and all had B waves in more than 50% of the monitored time. Conductance to cerebrospinal fluid outflow, measured by a lumbo-lumbar perfusion method, was significantly reduced: 0.042 ml X mm Hg-1 X min-1 (+/- 0.004 [SEM]; normal, more than 0.080 ml X mm Hg-1 X min-1). Cerebral blood flow was measured by xenon 133 inhalation and single photon emission computer tomography. Mean hemispheric flow was normal in all cases, averaging 59 +/- 9 ml X 100 gm-1 X min-1. Only 2 patients showed focal low-flow areas. Thus, a disturbance of cerebrospinal fluid circulation seems to be of pathogenetic significance in benign intracranial hypertension.