Evidence suggests that cats cannot eat grapes. Both cats and dogs can experience acute kidney failure from eating grapes. The link between grapes and kidney failure in dogs is well established, but is anecdotal with cats. Nevertheless, the cases of grape poisoning in cats suggest that they cannot eat them.
It is unknown why dogs and cats sometimes become fatally poisoned by eating grapes. Acute kidney illness strikes up to 72 hours after grape ingestion, and as little as 3 or 4 grapes can kill a twenty pound dog. Information on the amount that can kill a cat is scant due to the much lower number of reports of cat poisoning. This is likely due to cats being disinclined to eat fruit. Dogs are much more omnivorous than cats, and therefore more likely to consume grapes in the first place. Cats cannot even taste the sugar in fruit, and have little to no interest in it.