We have studied the centromere separation sequence in lymphocyte mitoses of the parents of four infants with trisomy 18, five patients with trisomy 21, and five children with normal karyotype. "Late separation" of chromosome 18 was found in both parents of a neonate with trisomy 18, "early separation" of chromosome 21 in three mothers and in one father of four children with trisomy 21. No "out-of-phase" separation occurred in the mitoses of the parents of normal children. The findings provide further evidence for the correlation between alteration of the parental centromere separation sequence and aneuploidy of the offspring.