A reduction in the proportion of the arterial inflow drained by the superior sagittal sinus in idiopathic intracranial hypertension (IIH) patients without stenoses has been noted and this has suggested elevated collateral flow. This study defines the interaction between arterial inflow and venous outflow in patients with outflow stenoses and IIH. Forty patients with clinical IIH underwent standard MRI, MR venography and MR flow quantification studies of the cerebral arteries and veins. There were 21/40 patients with venous stenoses. The arterial inflow was 21% higher than normal (p=0.01); however, the superior sagittal sinus outflow was normal, giving a reduced percentage of venous outflow compared to inflow. Seven patients were followed up after remission of their symptoms and the arterial inflows and percentage outflow returned to normal. There is a spectrum of findings in IIH; patients with stenoses have lower inflows than those with patent sinuses but still have evidence of collateral flow.