Hyponatremia is a common electrolyte abnormality among children and adults. Visual disturbance associated with hyponatremia has been described in the past; however, all of these cases presented primarily with other classical signs and symptoms of hyponatremia, such as seizures, gastrointestinal upset, lethargy, or headache. We present a child who developed blindness on post-operative day #6 after resection of a brain tumour. Computed tomography of the head showed no new bleed, mass lesion, edema, or occipital lobe pathology. Laboratory evaluation revealed acute hyponatremia (115 mmol/L). No other obvious symptoms of hyponatremia were present prior to its identification. As her hyponatremia was gradually corrected, her blindness completely resolved. This is the first reported patient with acute hyponatremia to present primarily with blindness; a "precondition" of chronically increased intracranial pressure (first hit) compounded by acute hyponatremia (second hit) may have induced reversible visual loss. Checking electrolytes should be the standard of care for neurosurgical patients with acute visual disturbance or other unexplained symptoms. It is a simple measure that quickly identifies a dangerous but reversible condition.