Stevens-Johnson syndrome: A serious systemic (body-wide) allergic reaction with a characteristic rash involving the skin and mucous membranes, including the buccal mucosa (inside of the mouth), conjunctiva, and genital areas. Abbreviated SJS. The disease is due to a hypersensitive (allergic) reaction to one of a number of immunologic stimuli including drugs and infectious agents. Complications can include hepatitis, nephritis, gastrointestinal bleeding, pneumonia, arthritis, arthralgia, fever, and myalgia. The diagnosis of SJS is usually made when the characteristic rash appears 1 to 3 weeks after exposure to a known stimulus and it cannot be explained by another diagnosis. The treatment depends, in part, on the suspected precipitating cause. Also known as erythema multiforme major.