Prenatal cytogenetic analysis of cultured amniocytes was performed after an increased foetal nuchal translucency thickness was detected by ultrasound in week 17 of a pregnancy. Analysis of GTG-banded chromosomes showed a small marker chromosome in six of the 12 colonies analysed. The supernumerary abnormal chromosome appeared to be positive with DA/DAPI staining and C-banding. The parents' karyotypes were normal. Using microFISH and FISH with band-specific probes, we found the marker appeared to be derived from chromosome region (16)(p13.1-->q12.2). Accurate identification of the marker chromosome was important for prenatal counselling: the marker chromosome contained euchromatic sequences, the foetus was carrying mosaic trisomy 16, and based on the literature the prognosis for the foetus was unfavourable and the pregnancy was terminated.