Six clinical observations of a rare condition, delayed radiation-induced optic neuropathy (RON) are presented. RON developed in patients with brain tumors treated by radiotherapy and radiosurgery; in the majority of patients the condition developed during exposure to therapeutic doses; its incidence was 0.5% of the total number of patients treated by radiotherapy during this period. Asymmetrical chiasmal syndrome developed rapidly in all the patients. Clinical diagnosis of RON was confirmed by magnetic resonance tomography in all cases, and in one case by morphological analysis. Conservative therapy including hyperbaric oxygenation just stabilized the visual function. Remote period of observation was 42 months.