Magnetic resonance imaging (MRI), after intravenous administration of the paramagnetic agent gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), showed enhancement of the optic nerve in 7 of 13 patients with acute optic neuritis. Four of these patients had Gd-DTPA enhancement of the intracranial optic nerve and two had involvement of the optic nerve at the orbital apex, ipsilateral to the eye with visual loss. Gadolinium-DTPA enhancement of both intracranial optic nerves was seen in one of two patients with bilateral retrobulbar neuritis. Two patients with acute radiation-induced optic neuropathy also had Gd-DTPA enhancement of the intracranial optic nerve. The authors recommend MRI with Gd-DTPA as the neuro-diagnostic procedure of choice for visualization of increased permeability of the blood-brain barrier in acute optic neuritis and radiation-induced optic neuropathy.