BACKGROUND: Radiation optic neuropathy (RON) is a rare, unpredictable, late complication of radiotherapy secondary to obliterative endarteritis. Tumor recurrence has to be ruled out by a clinical and neuroradiological examination. METHODS: Five patients with RON were investigated by magnetic resonance imaging (MRI) during 1992. RESULTS: Radiation-induced lesions of the intracranial visual pathways were easily visible on MRI. Without Gadolinium, a sectorial swelling was detectable, which markedly enhanced with Gadolinium. Intracranial optic nerve was affected in 5/5 cases, optic chiasm in 3/5 cases, and optic tract in 2/5 cases. CONCLUSIONS: MRI is the examination of choice when RON is suspected: it will easily delineate the extent of the lesion, and compression/infiltration by a recurrent tumor will be formally ruled out. A segmental swelling of visual pathway with marked Gadolinium enhancement on MRI is highly suggestive of radionecrosis.