The association of congenital chylothorax and trisomy X is important for counselling families at risk.Recently we treated a female newborn with congenital chylothorax who was thought initially to have a bilateral fetal hydrothorax.The baby was born by caesarean section at 35 weeks gestation due to fetal distress and hydrops fetalis. The mother was 42 years of age, her pregnancy was complicated by gestational diabetes and because of her age, an amniocentesis was performed in the 19th week. The fetal karyotype was 47XXX or trisomy X. This was con®rmed by karyotyping the baby's lymphocytes and then the ®broblasts when the diagnosis of chylothorax was established. The congenital chylothorax was treated according to the numerous articles found in the literature. The baby is seen regularly at the outpatient clinic and her clinical condition and development are both good. However, we could not ®nd any article in the literature concerning the association of congenital chylothorax and trisomy X.In an attempt to explain this case, we suggest that the fetal karyotype was a mosaic 47XXX/45X. Therefore in early fetal life there must have been a loss of one cell line, the 45X. This line, not detected later in chromosomal studies done in the amniocytes and the baby's cultured skin ®broblasts, was related to problems in lymphatic development resulting in the chylothorax.Most cases of chylothorax are sporadic, however Reece et al. [2] suggested an X-linked recessive transmission and another report [1] described a congenital chylothorax in two consecutive female siblings suggesting autosomal recessive inheritance. We believe that the association of congenital chylothorax with trisomy X is worthy of notice in order to counsel parents after the appearance of an aected ®rst child. References 1. King PA, Ghosh A, Tang MHY, Lam SK (1991) Recurrent congenital chylothorax. Prenat Diagn 11: 809±811 2. Reece EA, Lockwood CJ, Rizzo N, Pilu G, Bivicelli L, Hobbins JC (1987) Intrinsic intrathoracic malformations of the fetus: sonographic detection and clinical presentation. Obstet Gynecol 70: 627±632 Eur J Pediatr (2001) 160: 743