• Two patients with a heretofore undescribed unilateral choristomatous malformation of the intracranial optic nerve and chiasm underwent craniotomy for partial resection of the optic nerve in the belief that the tumor was either a glioma or a meningioma. Visual acuity in the affected eye was reduced to 20/200 in one patient and to finger counting in the other. Chiasmal involvement in the first case was associated with a superotemporal field defect in the opposite eye. Ophthalmoscopy disclosed atrophic nerve heads in both affected eyes. Heteroplastic masses of smooth muscle and adipose tissue replaced most of the resected optic nerve in each case.