White cats with blue eyes are usually deaf.
Pure white cats only make up about 5% of the overall cat population. Of these white cats however, 72% are deaf. In cat genetics, the gene that gives a pure white cat its white fur is also linked to the development of its ears and eyes. This is especially important for white cats with blue eyes.
More frequently than any other eye color, 65-85% of blue eyed white cats are deaf. Blue eyes and deafness are closely related in white cats, a trait observed by Charles Darwin in his 1868 book The Variation of Animals and Plants Under Domestication. Even more interesting, in heterochromatic white cats (cats with two different eye colors) when one eye is blue the ear on that side of the head will most likely be deaf. The non-blue eye’s side of the head will have normal hearing.
This is only true of pure white cats, those with entirely white fur. Cat’s that are mostly white but have colored markings, pointed patterns, etc. don’t count as being pure white cats and there is no connection between their eye color and their hearing. Siamese cats for instance are mostly white but they aren’t pure white cats and so there is no genetic relationship between their blue eyes and their hearing.
Added info: Interesting side fact, the pigmentation in the coat of Siamese cat is heat sensitive and changes color based on the temperature. The colder parts of their bodies (their extremities: feet, ears, nose, tail) are usually darker while the warmer parts of their bodies are lighter. If you send a Siamese cat outside in the cold winter months they will get darker, and upon returning to a warm house they will get lighter.
Also, in the 1970 Disney film The Aristocats the mother cat Duchess and her kitten daughter Marie beat the odds and are both white cats, with blue eyes, but normal hearing.