BACKGROUND: We have examined 18 children with a similar laterothoracic exanthem that appears to represent a distinct entity. OBJECTIVE: Our purpose was to describe the characteristic signs and clinical course of this eruption and its epidemiology data. METHODS: We observed the clinical course of the eruption in each child. RESULTS: The eruption has characteristic features. It occurs in a homogeneous age group (mean 23.3 months). It is initially unilateral and localized close to the axilla. The basic lesion is eczematous or scarlatiniform. The eruption evolves in two phases: it spreads centrifugally during the first 8 days and becomes more widespread on the tenth to fifteenth days, with predominant involvement on the half of the body initially affected. The lesions resolve spontaneously within 4 weeks. The long-term course is uneventful. CONCLUSION: The similarity of the cases suggests the existence of a new clinical entity. Many features favor a viral origin.