If only one pupil is dilated, you may have Adie's syndrome, a condition in which one pupil contracts more slowly than the other in response to light. It's usually caused either by a malfunction in the mechanism that controls the dilation reflex or from a harmless inflammation of the eye nerves.
More seriously, a single dilated pupil could indicate a brain injury, stroke or tumor.
If just one pupil is larger than the other, see the doctor immediately,
Depends.
1. large pupil reacts to light but not accomodation (gets larger @ smaller when finger test is performed) then 4getmenot is quite correct in that it can be Adie-Holmes Syndrome...not serious.
2. large pupil reacts to accomodation but not light...?could suggest underlying disease.
3. large pupil reacts to light and accomodation...normal
When I had Iritis in one of my eyes one of the side effects was that the pupil was larger than in the other eye. I think it was just because the eye wasn't filtering the light properly, it just wasn't dilating.
Was easily fixed with a course of eye drops but she shouldn't hang around, get to the docs asap. In fact she'd probably better just going straight to the eye hospital as that's more than likely what the gp will advise.