PallisterKillian syndrome (tetrasomy 12p) is a relatively rare aneuploidy syndrome characterized by the presence of mosaicism for an isochromosome 12p [i(12p)]. We report two new cases diagnosed following chorionic villus sampling and an abnormal ultrasound, respectively. Fluorescent in situ hybridization (FISH) was used to enumerate the number of interphase cells containing the isochromosome. The results of these studies illustrate the importance of the use of interphase FISH to detect the presence of the i(12p) in uncultured, non-dividing cells. A review of the literature identified 23 additional cases of PallisterKillian syndrome diagnosed prenatally. Approximately 50 per cent of these cases were associated with the presence of a congenital diaphragmatic hernia. We suggest that a perinatal-lethal form of PallisterKillian syndrome is underdiagnosed and recommend that all cases of prenatally detected diaphragmatic hernia be tested for PallisterKillian syndrome using interphase FISH on uncultured amniocytes. © 1997 by John Wiley & Sons, Ltd.