Monochorionic twins were phenotypically discordant for Down syndrome. Chromosome analysis of lymphocytes showed both to be mosaics. Fibroblast chromosome analysis showed that one twin was 46,XY and the other 47,XY, +21. Blood mosaicism appears to have resulted from placental anastomoses. The twins are presumed to be monozygotic on the basis of placentation, identical blood groups, HLA haplotypes, and serum proteins. We offer some speculation on the manner of occurrence of this rare event.