An 11-year-old male child presented with worsening vision in his left eye. Examination and imaging revealed a left optic nerve tumour causing mass effect and optic neuropathy, without systemic evidence of neurofibromatosis. In view of the significant risk to visual acuity, a biopsy was deferred and chemotherapy was commenced. After initial stability, continued visual decline necessitated incisional biopsy. Surgical navigation was used to facilitate minimal access surgery avoiding bone removal. The system also precluded biopsy of cystic parts of the tumour, allowing successful intraoperative frozen-section confirmation of lesional tissue.