The petpoisonhelpline.com says YES. They are extremely poisonous to cats, and just 1-2 leaves or even pollen can kill a cat. Common signs of poisoning develop within 6-12 hours of exposure, and include vomiting, lack of appetite, lethargy, and dehydration. Signs of not urinating or urinating too frequently, not drinking or excessive thirst should be taken seriously. Antidote and treatment must happen quickly. Other plants with lily in the common name are not related to lilium, and do carry the same threat.
Here’s Baxter contemplating our question of the day.