A round hole in the center of the iris, called the pupil, narrows normally under bright light stimuli and expands significantly under low light as a way to regulate the amount of light entering the eye. Unequal pupil size in both eyes may be caused by unilateral dilatation or narrowing of the pupil. A dilated pupil in one eye may be caused by glaucoma, glaucoma ciliary syndrome, post-traumatic eye injury, paralytic pupil monotony, intraocular foreign bodies, retinoblastoma, etc. The use of eye drops to dilate the pupil may also cause this, and it may be related to systemic poisoning or oral medication. Pupil narrowing may be caused by iridocyclitis, keratitis or post-traumatic irritation, or by local or systemic medications. In addition, some patients with central nervous system injury and diabetes may also experience pupil narrowing. Therefore, unequal pupil size may be related to many diseases and it is recommended to visit the local hospital ophthalmology clinic in a timely manner.