The so-called puppy lemon law is designed to undercut what animal-rights activists call "puppy mills." These are big operations in which dogs are often kept in squalid conditions.
When the resulting puppies show up in pet stores, unsuspecting buyers can bring home dogs with diseases or genetic problems.
Vicki Deisner is with the American Society for the Prevention of Cruelty to Animals, which supports the law. She says most people who buy a sick puppy are likely to want to keep it and care for it -- and the new law is meant to help them pay for that care up to the purchase price.
But if they do take the return option?
"To be honest, I really don't know what happens with pet store dogs. But I do know that in puppy mills, things can happen to the degree that sometimes they just go out and drown an animal because they won't even waste the money for a bullet to shoot it." -Vicki Deisner, American Society for the Prevention of Cruelty to Animals.
The text of the puppy lemon law is silent on what happens to the dogs. Some activists say it's better to lose one puppy if it prevents the spread of disease to many others.
Illinois Public Radio's Brian Mackey contributed to this report.