When Your Cat’s Pupils Become Two Different Sizes

Sometimes there’s nothing you have to do about it; sometimes there is. But you need a veterinarian to weigh in.

Published: March 24, 2025

Screenshot — Courtesy of Dr. Jaycie Riesberg, Diplomate of the American College of Veterinary Ophthalmologists As you can see, the pupil on the left is significantly smaller than the one on the right.

You notice that the pupils—the black circles in the center of your cat’s eyes—are no longer the same size. One has become significantly larger than the other, blocking out much of the iris (the colored part). Or one has become significantly smaller. Either way, it’s not normal. Both pupils are supposed to enlarge to the same degree to allow more light in when the ambient light has dimmed, and they’re supposed to shrink to the same degree to let in less light when the surrounding light has brightened. The condition of one pupil remaining a different size from the other no matter what is called anisocoria, from the Greek aniso, meaning “unequal,” kore, meaning pupil, and the Latin suffix ia, signifying “abnormal.”