The hamartoma of the tuber cinereum, a midline dysraphic syndrome, corresponds to a mass of normal neuronal tissue implanted in the area of the mamillary bodies. It has no evolutionary potential, but association to other cerebral malformations has been observed. On the basis of 18 personal cases and a review of the literature, the authors evaluate the actual diagnostic criteria and the incidence of different clinical signs such as precocious isosexual puberty, seizures, and mental impairment.