A diagnostic pearl: the school chair sign.

Allergic contact dermatitis (ACD) reactions to nickel have pleomorphic presentations owing to the ubiquity of this allergen in the environment. In children, ACD to nickel classically occurs on the earlobes, neck, wrists, and infraumbilical area. We describe a newly recognized presentation for this common childhood ACD--on the posterior thighs--which we have termed the school chair sign.