All parts of the lily plant (leaves, petals, bulbs, even the pollen!) are considered toxic to cats and consuming even small amounts can be life threatening. Within a few hours of ingestion of the lily plant, a cat may vomit, become lethargic or develop a lack of appetite. These signs continue and worsen as kidney damage progresses. Without prompt and proper treatment by a veterinarian, the cat may develop kidney failure in 36 to 72 hours.
I continue to be surprised at the number of people who do not know that the Easter Lily is one of the most toxic plants to cats, and how the plants are not labeled with warnings! This time of year, every grocery store has huge displays of Easter candy, baskets, egg decorating kits and yes, Easter Lilies...all ready to be put in the homes of unsuspecting cat owners!
There are many nice alternatives to having lilies in the house (most lilies are toxic to cats, but not seen inside homes as often). Orchids, Easter cactus and violets are options, as are lovely artificial silk lilies.
Here is a link to more info: