Primary optic nerve gliomas are most commonly benign and occurring in childhood. Malignant optic gliomas occur in adulthood and follow an aggressive course usually leading to blindness and death within months. This paper describes the clinical and histopathological features of clinically aggressive but histopathologically benign optic nerve gliomas of adulthood (a third category of optic glioma). Three cases of this rarely reported finding were selected for detailed study, including fundus photographs, visual field reports as well as MRI scan results. In all three cases, patients were initially diagnosed with optic nerve inflammation or benign neoplasm. They all went on to develop neuroradiological extension and all had a rapid deterioration of vision in the affected eye. Visual acuity deteriorated to no perception of light over a three to eight week period. The findings from these cases indicate that benign optic nerve gliomas in adults can be misdiagnosed as inflammation, be clinically aggressive and require excision to prevent further intracranial involvement.