Q. I have a 6 yr, old, neutered, male tabby cat. His left eye is dilated and his right eye is normal. No recent meds. He is eating and acting normal.
DR. CHRISTIE LONG, D.V.M., C.V.A.
A. You're describing something called "anisocoria", which means unequal pupil sizes. Sometimes as cats age their iris (the colored part of the eye) atropies and we'll see one pupil take on a slightly different shape or appearance. But there are some concerning problems that can cause anisocoria, including certain inflammatory diseases, diseases of the neurologic system (brain or nerves controlling pupil size) and exposure to some drugs (unlikely in this case since you mentioned that he hasn't been given any drugs).
I'm glad that he seems normal otherwise, but see a vet so that the eyes can be examined and you can make sure this is not a serious condition.