Very. I found out yesterday. I received a bunch of lilies for my birthday last Thursday, my cat was severely ill Thursday night. My partner and I didn't know what it was and we thought it would pass, but she just got worse. She was finally diagnosed yesterday. He kidneys have failed. She has been in the vet for 2 days now, she'll be there tomorrow and then she'll be re-tested. It's not looking good. She's my partners best friend, she's like our baby and she was, until last Thursday, a playful, happy, mischievous cat. Now she won't move off my pillow on the bed, won't smile, won't eat or drink or go to the toilet. We have spend every cent we have and more. About $1000.00 so far in 3 days. I can't believe I didn't know lilies were lethal to cats. I wish I was you, whoever you are, asking this question 3 weeks ago. My cat wouldn't be dying.
Yes they are very toxic the leaves etc to kittens who chew and the pollen to older cats , always check your plants etc, even if they survive lilly poisoning they can still have serious long term health problems renal failure etc