This report serves to emphasize the necessity of rapid cytogenetic testing during fetal therapy for congenital hydrothorax and to review the literature. A 31-year-old primigravid woman was noted to have bilateral fetal hydrothorax, polyhydramnios, and preterm labor at 32 weeks' gestation. Echo-guided thoracocentesis was performed to draw 50 ml of golden/yellow pleural effusion fluid and 500 ml of amniotic fluid. Cytogenetic analysis of the lymphocytes obtained from the pleural effusion fluid revealed a karyotype of 47, XY, + 21. The pleural effusion fluid was predominantly lymphocytic and positive for the Rivalta test. A sonographic examination at 33 weeks' gestation revealed recurrent pleural effusion, but the woman refused repeat thoracocentesis and tocolytic management. A 2,568-g male baby with characteristic phenotypic findings of Down syndrome was delivered vaginally and expired after birth. The present case reinforces the notions that fetuses with congenital chylothorax are at risk for chromosomal abnormalities, and drainage of pleural effusion must include a rapid diagnosis of fetal karyotype. The cytogenetic information acquired is useful for genetic counseling and perinatal obstetric management.