This report documents the long-term clinical and histopathological behaviour of eight intraorbital and 16 intracranial optic nerve gliomas and relates the therapeutic data to the prognosis for both visual acuity and survival. The mean age at onset of symptoms was 8.6 years and at the diagnosis 10.9 years. It is generally held that proptosis is mild in intraorbital glioma, but we encountered marked proptosis ranging from 7 to 12 mm in six of the eight intraorbital gliomas, which contained abundant Alcian-blue-positive mucoid material. Of the patients with intracranial optic nerve gliomas 37.5% survived for a mean of eight years after treatment with radiotherapy or surgery combined with radiotherapy. At follow-up ranging from five months to 11 years only one of the six patients with intracranial gliomas had full visual acuity. Our observations emphasise that, although optic nerve gliomas are benign hamartomas, the prognosis for visual acuity and survival is unfavourable in cases which are diagnosed and treated late. Histopathological and histochemical observations suggest that increase in the amount of mucoid material may contribute to rapid enlargement of intraorbital and intracranial optic gliomas.