We describe the combined use of comparative genomic hybridization (CGH) and fluorecence in situ hybridization (FISH) to identify the origin of de novo unbalanced translocations in a fetus with abnormalities on ultrasound examination and in a newborn with multiple congenital abnormalities. RHG banding of amniocytes and lymphocytes respectively showed a unbalanced karyotype: 46,XX,add(4)(q34), with normal parental karyotypes in both cases. CGH revealed a gain of material from distal 15q (q23qter) in the fetus and a gain of distal 7q (q31qter) in the newborn. CGH results were confirmed using FISH with painting probes in both cases. These cases demonstrate the efficiency of CGH in identifying the chromosomal origin of extramaterial in unbalanced de novo translocations.