Chromosome abnormality associated with the clinical syndrome of mongolism can usually be attributed to primary non-disjunction during gametogenesis. Nevertheless, a variety of chromosome abnormalities not attributable to primary non-disjunction during gametogenesis have been described, usually in mongols born to young mothers. The authors of this paper report a 48/46 chromosome mosaicism in a mongol born to a young mother. The cell line with 48 chromosomes has five chromosomes of the 21–22 group (Denver system) and a centric fragment. The cell line with 46 chromosomes has an extra chromosome with the morphology of the 19–20 group and only three chromosomes of the 21–22 group. It is suggested that this mosaicism resulted from concurrent non-disjunction of a number 21 and transverse division of the centromere of a second number 21 during early cleavage in a zygote, or cell line, with 47 chromosomes and trisomic for chromosome number 21.