Fryns syndrome is a lethal autosomal recessive multiple congenital anomaly syndrome characteristic "coarse" facies, cleft palate, diaphragmatic hernia, and distal digital hypoplasia. The appearance of the face and digits is very similar to that observed in Pallister-Killian syndrome (mosaic isochromosome 12p), although the incidence of cleft palate, diaphragmatic hernia, and neonatal death is much lower in the latter condition. We report on an infant with many manifestations of Fryns syndrome ("coarse" face, cleft palate, cloudy corneae, diaphragmatic hernia, distal digital hypoplasia, and neonatal death) who was found to be mosaic for i(12p). Her diagnosis was changed to Pallister-Killian syndrome and the family was counselled accordingly. The clinical overlap between Fryns and Pallister-Killian syndromes is discussed. Because the chromosome abnormality in Pallister-Killian syndrome is often limited to fibroblasts and may be selectively eliminated both in vivo and in vitro, some Pallister-Killian patients may be misdiagnosed with Fryns syndrome and given an erroneously high recurrence risk. Newborn infants with the Fryns or Pallister-Killian phenotypes should have chromosome studies involving multiple tissues so that the correct diagnosis can be made. This will contribute to the understanding of both disorders and facilitate appropriate genetic counselling.