Cavernous angiomas of the cranial nerves are extremely rare, and those of the oculomotor (third) nerve are rarer still. The authors present a single case study of presumed cavernous angioma involving the subarachnoid portion of the left third nerve, which is the first reported case found in the literature. The patient is a three-month-old male infant who presented as an acute left third nerve palsy. He presented with acute onset of left eyelid ptosis, decreased extraocular motility with an associated left exotropia and hypotropia, and anisocoria with the left pupil larger and less reactive than the right. Magnetic resonance (MR) imaging revealed enlargement of the initial subarachnoid portion of the left oculomotor nerve. The child was followed without intervention and the palsy completely resolved. The authors report that it is therefore reasonable to follow patients with these lesions without intervention, given the poor functional outcome results with attempted surgical excision and the potential for spontaneous improvement.