A predominantly unilateral hydrothorax in an unborn child was diagnosed by sonography during the 34th gestational week. Because of a pathologic cardiotocogram it was necessary to make a primary caesarean section one day later. The premature baby was intubated immediately and the pleural discharge was removed. Later developmental of the child was uneventful. The hydrothorax was interpreted as a partial factor of a hydrops fetalis. Controlling the fetal pleural discharge by ultrasound examination and regular cardiotocogram a caesarean section should be done between the 34th and 36th week of pregnancy.