We present a case of a liveborn male with complete trisomy 1q in mosaic form due to a de novo unbalanced translocation. There are seven previously documented cases of complete trisomy for 1q, which demonstrate that this is a lethal condition. All cases have similar phenotype including weights greater than 50th centile for gestational age, hydrocephalus, microphthalmia, abnormal ears, small mouth or jaw, and abnormal fingers. Single umbilical artery, imperforate anus, and dysplastic kidneys are also seen in more than one patient. Five of the eight translocation cases have identical chromosomal breakpoints involving 1q and Yq. This suggests the possibility of sequence similarities on these two chromosomes as has been documented with several other recurrent chromosomal rearrangements. Further, this case demonstrates the need for postnatal genetics evaluation following prenatal diagnosis. In postnatal testing, the aneuploidy could not be demonstrated in metaphase cells from cultured lymphocytes. More detailed testing prompted by abnormal amniocentesis and neonatal dysmorphology was necessary to confirm the cytogenetic diagnosis. Without the prenatal diagnosis, it is likely that the true cytogenetic aberration would have gone undetected. © 2005 Wiley-Liss, Inc.