Why do grapes cause kidney failure in dogs?

The cause of kidney failure in dogs due to grapes and raisins is still unknown. It is known, however, that the danger occurs with both commercially-grown and home-grown grapes, so it is probably something in the grapes themselves rather than an insecticide or something like that. Raisins are even more toxic.

One or two grapes is no big deal, unless it's a small dog. More than that, or any amount of raisins, is a reason for a quick trip to the vet. The sooner support is initiated, the more likely a successful treatment.