An acute and severe skin disease with clinical and histological features of destruction and detachment of the skin epithelium and mucous membranes. Onset may occur at any age, but the risk increases after 40 years. Three subforms have been described according to the percentage of the body surface area affected: Stevens-Johnson syndrome (less than 10%), Lyell syndrome (greater than 30%) and an intermediate form (10-29%). The initial manifestations are nonspecific: a seemingly banal rash, fever, and a burning sensation involving the eyes, mouth and genitalia. The rash rapidly progresses to become vesicular and bullous on the face and body. High fever is a constant feature. Two thirds of cases are triggered by a drug allergy. In rare cases, the disease is associated with infections or bone marrow transplantation. The remaining 25-30% of cases are classed as idiopathic. The prognosis for patients with extensive forms is poor.