Grapes and Raisins can cause irreversible damage to the kidneys, possible resulting in death.
Ingesting as few as 4-5 grapes or raisins can be poisonous to a 20 pound dog, though the exact toxic dose is not established.
Signs of toxicity include vomiting, loss of appetite, diarrhea, abdominal pain, decreased urine production (possibly leading to lack of urine production), weakness and drunken gait.
Onset of signs typically occurs within 24 hours (though they can start just a few hours after consumption)
Your vet may start by inducing vomiting, or the stomach might be pumped (gastric lavage