We describe a patient in whom full monosomy 21 was initial ly assumed from routine GTG banded karyotyping. Re-examination with chromosome painting demonstrated an unbalanced translocation between the long arms of chromosomes 11 and 21. Fluorescence in situ hybridization (FISH) and microsatellite marker analysis revealed partial monosomy of chromosome 21 (pter-q22.2) and 11 (q24-qter). The patient was prematurely born in the 31(st) week of gestation and expired 3 days after delivery. She showed multiple minor anomalies, a complex cardio-vascular malformation, intestinal malrotation and cerebellar hypoplasia.