A patient with nevus unius lateralis, brain tumor, and diencephalic syndrome of childhood is presented. The diencephalic syndrome is a rare condition occurring in very young children manifested by failure to thrive and emaciation. It is almost invariably caused by an astrocytoma of the anterior hypothalamus. While there are many dermatological conditions associated with neurological abnormalities, an association between nevus unius lateralis and a neurological disorder has not previously been reported. Whether this case represents such an association or merely the chance occurrence of these two entities in the same patient is unknown at this time.