Heterotopias of well-formed neurons with or without undifferentiated neuroblastic cells were found in periventricular areas of the brain in 13 of 16 infants with trisomy 18 syndrome. These heterotopias were located most commonly above or lateral to the head of the caudate and/or in roof and lateral wall of inferior and posterior horns, within fibers converging on the internal capsule, and of the optic radiation. These cerebral heterotopias seem to stem from excessive formation of embryonal neuroblasts with focally arrested migration in the periventricular white matter, without resulting in deficient neuronal composition of the cerebral cortex. Similar heterotopias were present in two cases of trisomy 18 phenotype with a normal karyotype. As no such cerebral heterotopias were thus far observed in the trisomy D and G syndromes, they appear as a comparative diagnostic stigma of trisomy 18.