A 16-year-old woman complaining of headache and declining vision in both eyes had papilledema, normal brain imaging, and a lumbar puncture showing a moderately high opening pressure (35 cm H2O) and normal cerebrospinal fluid constituents. For a diagnosis of idiopathic intracranial hypertension (IIH), she was treated with acetazolamide and methylprednisolone, but vision worsened, so she underwent bilateral optic sheath fenestration (ONSF). Within the 1st postoperative week, vision had improved and papilledema was less prominent. However, by the 14th postoperative day, vision had worsened and headache persisted. Lumbar puncture showed a very high opening pressure (65 cm H2O), so she underwent ventriculoperitoneal shunting. Although there was a slight initial improvement in vision, it eventually declined further. This case emphasizes that ONSF may yield initial improvement in vision and reduction in papilledema yet not prevent eventual visual loss in IIH. Whether the visual loss in this patient resulted from persistently elevated intracranial pressure after ONSF or was prefigured before ONSF occurred is unresolved. It is a reminder that patients with IIH must be monitored carefully after ONSF. If there is a suggestion of further visual loss, shunting should be considered if intracranial pressure is high.