Magnetic resonance (MR) imaging was performed on six patients with idiopathic intracranial hypertension (IIH). The patients were all women with a mean age of 27 years (range, 17-41 years). Six healthy women volunteers with a mean age of 33 years (range, 25-46 years) were studied for comparison. All six patients had elevated opening pressures at lumbar puncture. MR imaging demonstrated a partially empty sella in two patients, and one patient had signal aberration in the dural venous sinuses suggestive of slow flow. There were no white matter signal abnormalities, even in the periependymal space. Volumetric pixel analysis revealed significantly larger subarachnoid space volumes in patients than in control subjects, without a significant difference in ventricular volumes. No other intracranial abnormalities were found in any IIH patient or control subject. The results suggest that, in IIH, (a) although the subarachnoid spaces are enlarged, the wide range of normal values would limit the clinical use of these measurements and (b) the role of MR imaging, as with CT, is primarily in the exclusion of other diseases with clinical presentations similar to that of IIH.