Are raisins harmful to dogs?

Yes, they can be very toxic, causing renal failure. This is a very handy guide to dangerous foods. I keep meaning to put a copy up on my fridge for reference.

Yes, raisins, grapes, onions, macadamia nuts, xylitol (in gum, mints, and even some peanut butter now) are all harmful to dogs.

Grape and raisin toxicity is well documented in dogs. Although the exact substance that causes the toxic reaction is not yet known, dogs should not eat grapes and raisins because even small amounts can prove to be fatally toxic for a dog.

Dogs of any age, breed, or gender may be affected. Grapes and raisins are bad for dogs because one of the most serious complications of grape/raisin toxicity is acute (sudden) kidney failure with lack of urine production (anuria). However, kidney failure is not seen in all dogs after ingestion of grapes or raisins, and again, the reason why some dogs are affected excessively while others are not is still being studied.

Yup! Don't give it to your dogs, definitely not good for them!