The authors present a case of partial trisomy 12q associated with chylothorax, diagnosed at 30 weeks of pregnancy. Cordocentesis for the karyotype as well as thoracocentesis were carried out. In spite of the administration of tocolytic drugs the patient delivered a girl with multiple clinical abnormalities, who died at 7 days of age. From this case, the authors report 6 other cases of partial trisomy 12q in the literature, and, in the discussion they suggest the management after the diagnosis of hydrothorax has been made by ultrasound.