Easter lilies have been found to be toxic to cats. The primary toxic effects are on the kidneys. Within minutes to hours of ingesting some of the lily plant, a cat may stop eating, begin vomiting, and become dehydrated and lethargic. If no treatment is given, rapid and severe kidney failure occurs in a matter of days. All parts of the lily are considered toxic. If your cat has ingested part of a lily plant, rapid veterinary care is strongly recommended.
For more information, please read the story Easter Lilies - Not Just a Pretty Flower.