Data from the New York State Chromosome Registry on over 10,000 cases of Down syndrome reported from 1977 to 1996 confirm findings in the England and Wales Cytogenetic Register that in mosaic 46/47,+21 cases of Down syndrome the male/female ratio (as inferred from XY and XY karyotypes respectively) is less than 1.0 as opposed to the ratio in nonmosaic 47,+21 cases in which the ratio is close to 1.2, or in the general background population with ratio of about 1.05. These results may reflect in part differential pairing in 47,+21 somatic cells of X and Y chromosomes with a 21 chromosome and/or in in-utero selection.