Management of patients with idiopathic intracranial hypertension (IIH) should be based on the presence and progression of visual loss. To characterize the clinical course of IIH more completely, we monitored the clinical status, especially visual function, in 50 consecutive newly-diagnosed patients over a period of 2 to 39 months (average follow-up 12.4 months). The mean age at onset of symptoms was 31 (range 11-58) yrs; 46 (92%) were women and 47 (94%) were obese (mean weight 90 kg). Common symptoms were headache (92%), transient visual obscurations (72%) and intracranial noises (60%); 13 of the patients (26%) initially had complaints of sustained visual loss. There was visual loss as determined by Goldmann perimetry in 96% and by automated perimetry in 92%. Contrast sensitivity testing was abnormal in 50% and Snellen acuity in 22%. Two patients (4%) became blind in both eyes. The Goldmann visual field grade improved in 60% of patients but visual function deteriorated in 5 (10%). Deterioration of visual field grade was significantly associated only with weight gain during the year before diagnosis. Visual loss in patients with IIH is common and is often reversible. Patients should be evaluated by perimetry using an appropriate strategy and contrast sensitivity testing, along with careful examination of the optic discs.