Hydrops foetalis (HF) is defined as an accumulation of fluid in at least two compartments of the body, e.g. the subcutis, the pleural cavities, the pericardial cavity and the abdomen. In present time, non-immune causes, including tumours, predominate. We present a case of severe HF, caused by a mass which was clinically interpreted as a hernia of the diaphragm at gestational week 19 + 6. An abortion was granted and performed. Autopsy revealed an immature teratoma in the mediastinum compressing the heart and airway. The prognosis and treatment of mediastinal teratoma and HF is discussed.