Case Presentation: A 22-year-old female presented to the emergency department with a dilated right pupil and mild blurry vision. Physical examination revealed a dilated, sluggishly reactive right pupil, without other ophthalmic or neurologic abnormalities. Neuroimaging was normal. The patient was diagnosed with unilateral benign episodic mydriasis (BEM). 

Discussion: BEM is a rare cause of acute anisocoria with an underlying pathophysiology that is not well understood. The condition has a female predominance and is associated with a personal or family history of migraine headaches. It is a harmless entity that resolves without intervention and results in no known permanent damage to the eye or visual system. Benign episodic mydriasis is a diagnosis of exclusion that may be considered only after the life- and eyesight-threatening causes of anisocoria.