We identified distinctive and characteristic abnormalities in the hippocampal formations of 4 individuals with thanatophoric dysplasia (TD), one of whom was a fetus of 19 weeks gestational age. Primitive medial fissures in this subject could be identified, but development of the dentate gyrus and organization of the pyramidal layer in the hippocampal formation were abnormal. We infer that temporal-lobe dysmorphogenesis in TD begins between 11.5 weeks gestational age, when hippocampal and fimbriodentate fissure formation takes place, and 13.5 weeks gestational age, when differential proliferation and migration of the cells that form the pyramidal layer and the dentate gyrus takes place. Any etiology proposed for TD must not only account for disordered endochondral and endomembranous bone formation but also explain the origins of early temporal-lobe dysplasia.