A 61-year-old man presented with acute painless optic neuropathy with resultant no light perception in his left eye. Neuro-ophthalmological examination, optical coherence tomography and fluorescein angiography did not reveal the etiology. Since the patient had a cardiac pacemaker, he underwent a CT scan with contrast of the brain and orbits, which was normal. Five months later, the patient presented with visual field loss in his right eye. A repeat targeted CT scan was normal but after stopping his pacemaker, an MRI of the brain was obtained and revealed a space-occupying lesion involving the optic chiasm and both optic nerves. Lesion biopsy was consistent with glioblastoma multiforme. Despite treatment with radiotherapy and chemotherapy the patient died four months later. This case report emphasizes the importance of insisting on a high-quality brain MRI in the workup of optic neuropathy.