Abstract: Erythema dyschromicum perstans (EDP) is asymptomatic slate-gray to blue-brown macules. Idiopathic eruptive macular pigmentation is asymptomatic brown nonconfluent macules. We describe electron microscopic studies of a 9-year-old Japanese girl with EDP. The ultrastructural figures indicated that the production of immature, small, irregular-shaped melanosomes in melanocytes and peripheral localization of melanosomes in keratinocytes caused the clinical appearance of EDP. The ultrastructural evidence distinguishes EDP from idiopathic eruptive macular pigmentation and suggests a distinct pathogenesis of the disease.