A 2-year-old boy was admitted with an acute onset of generalized bullae. Before hospitalization, he had developed generalized erythema, become lethargic, and had an episode of vomiting and diarrhea. Within 2 hours, tense bullae had appeared, mostly on his head, neck, arms, and trunk. His only medication at the time of admission was dextromethorphan, for an upper respiratory tract infection. The findings of his medical history were unremarkable, except for occasional urticaria, which worsened with heat exposure.