A 6-year-old boy developed a unilateral itchy rash, which was followed by a less severe bilateral involvement. The lesions resolved spontaneously within 4 weeks. Apparently, the rash was triggered by an adenovirus infection of the respiratory tract. Terms used currently for this disorder include ‘unilateral laterothoracic exanthem’ or ‘asymmetrical periflexural exanthem of childhood’. The recently proposed term ‘superimposed lateralized exanthem of childhood’ implies the new concept that an early postzygotic mutation has rendered the skin on one side of the body more reactive to infective agents. The subsequent bilateral involvement would reflect a less severe reactivity of the contralateral keratinocytes.