I believe they have been known to cause renal failure. And it doesn't take a lot apparently.
As a weird side note: I watched a TV documentary one time about canines and it showed some coyotes feasting on someone's grape vines every night. So, I don't know what happened to the coyotes. They may or may not have become ill. No one on that documentary mentioned anything about the adverse health effects of grapes. Nevertheless, I certainly won't chance it with my dogs.