Anisometropia is a condition in which a person's eyes have substantially differing refractive power. Generally, a difference in power of one diopter (1D) is the threshold for diagnosis of the condition. Patients may have up to 3 diopters of anisometropia before the condition becomes clinically significant due to headache, eye strain, double vision or photophobia.

In certain types of anisometropia, the visual cortex of the brain cannot process images from both eyes simultaneously (binocular summation), but will instead suppress the central vision of one of the eyes. If this occurs too often during the first 10 years of life, while the visual cortex is developing, it can result in amblyopia, a condition where, even when correcting the refractive error properly, the person's vision in the affected eye may still not be fully correctable to 20/20.

The name of the condition comes from its four Greek components: an- "not", iso- "same", metr- "measure", ops "eye".

Antimetropia is a rare sub-type of anisometropia in which one eye is myopic (nearsighted) and the other eye is hyperopic (farsighted). This condition occurs in about 0.1% of the population.

Anisometropia is caused by common refractive errors, such as astigmatism, far-sightedness, and myopia, in one eye.

Anisometropia is likely the result of both genetic and environmental influences.

Some studies suggest, in older adults, developing asymmetric cataracts may cause worsen anisometropia. However, anisometropia is associated with age regardless of cataract development: a rapid decrease in anisometropia during the first years of life, an increase during the transition to adulthood, relatively unchanging levels during adulthood but significant increases in older age.

Anisometropia causes some people to have mild vision problems, or occasionally more serious symptoms like alternating vision or frequent squinting. However, since most people do not show any clear symptoms, the condition usually is found during a routine eye exam.