Unusual and virtually identical hypothalamic tumors were recently studied in three unrelated neonates with a similar complex malformation syndrome. Previous reports of either the hypothalamic tumors or the syndrome as a whole have not been found. Each tumor was apparent on the inferior surface of the cerebrum and extended from the optic chiasma to the interpeduncular fossa. The tumor replaced the hypothalamus and other nuclei which originate in the embryonic hypothalamic plate; it was principally composed of cells resembling primitive, undifferentiated germinal cells. The term "hamartoblastoma" is used to designate these tumors in order to emphasize the malformational and neoplastic aspects. In addition, short olfactory tracts suggest a relation to the arrhinencephaly field defect.