It has been postulated that cerebral venous outflow "obstruction" is a precipitating factor for many cases of idiopathic intracranial hypertension (IIH). We describe a 17-year-old woman with IIH, and "venous obstruction" repeatedly demonstrated on magnetic resonance venography (MRV) that within minutes resolved partially when the cerebrospinal fluid (CSF) pressure was reduced to 11 cmH(2)O and completely when the pressure was reduced to 8cmH(2)O. These findings further support the view that raised pressure is the cause of the "obstruction", rather than the obstruction being the primary cause of the IIH. It also raises questions about how low the CSF pressure should be reduced at therapeutic lumbar puncture.