We describe a white female infant with neurocutaneous melanosis (NCM) and encephalocraniocutaneous lipomatosis (ECCL). Multiple, giant and small congenital melanocytic nevi (CMN) were observed on the head, neck and trunk and involved 70% of body surface area. Histologic examination of several CMN revealed atypical nodular proliferations of dermal nevomelanocytes. In a small (<1 cm) truncal CMN, single and dyscohesive intraepidermal nests of atypical nevomelanocytes simulating a superficial spreading melanoma, were observed. The placenta was grossly normal and histologically demonstrated multiple banal appearing nevomelanocytes within the stroma of its villi. At the 17-month follow-up no evidence of primary or metastatic melanoma was present. This previously undescribed association of NCM, ECCL and placental nevomelanocytes provides strong support for the hypothesized causal role of anomalous neural crest morphogenesis and migration in the development of all three disorders. The genetic mechanism underlying these complex birth defects has been hypothesized to result from the action of lethal autosomal dominant genes surviving by mosaicism. (J Am Acad Dermatol 2002;47:S196-200.)Le texte complet de cet article est disponible en PDF.