Toxic Epidermal Necrolysis (TEN), also known as Lyell's syndrome, is a life-threatening dermatological condition that is frequently induced by a reaction to medications. It is characterized by the detachment of the top layer of skin (the epidermis) from the lower layers of the skin (the dermis) all over the body.

There is broad agreement in medical literature that TEN can be considered a more severe form of Stevens-Johnson syndrome, and debate whether it falls on a spectrum of disease that includes erythema multiforme.

The incidence is between 0.4 and 1.2 cases per million each year.