The nevus sebaceous of Jadassohn in combination with ocular abnormality, convulsions, and mental deficiency constitutes a well-defined though poorly recognized neurocutaneous syndrome. The cardinal feature of this entity, the nevus, is only part of an abiotrophy affecting ectodermal derivatives, specifically skin, eye, and brain. It is clear that the presence of an organoid nevus may serve as a sign of multiple ectodermal and mesodermal malformations. The importance of the nevus sebaceous of Jadassohn as a lesion which may undergo benign or malignant change is emphasized. Like neurofibromatosis (von Recklinghausen), tuberous sclerosis (Bourneville), angiomatosis retinae et cerebellae (von Hippel-Lindau), and encephalotrigeminal angiomatosis (Sturge-Weber), the nevus sebaceous of Jadassohn should be categorized as one of the phakomatoses, the term coined by van der Hoeve to signify "mother spot."