Twenty-five patients with abnormal blood vessels on the optic nerve head were examined both clinically and with fluorescein angiography to determine the diagnostic significance of these abnormal vessels. Six patients had congenital abnormal vessels--prepapillary or optociliary arteries or veins, or vessels associated with congenital anomalies of the disc. Nineteen patients had acquired abnormal vessels that were associated with retinal vein occlusion, chronic glaucoma, chronic papilledema, meningioma or vasculitis. The clinical and fluorescein angiographic features that differentiated these groups are discussed in detail.