We report a case of 45,XY,-5,-21,+der (5) t(5;21) (p13 or p14;q11.2 or q21) that was prenatally misdiagnosed as complete monosomy 21 and terminated at 24 weeks of gestation. Subsequent fluorescence in situ hybridization studies with a chromosome 21 painting probe documented the cryptic unbalanced translocation. (C) 1994 Wiley-Liss, Inc.