Purpose: Fetal immature mediastinal teratoma is a rare disease. The pressure generated by the tumor mass can cause hydrops fetalis, pulmonary hypoplasia, pleural and peritoneal effusion, and polyhydramnios which cause the death of the fetus. Routine prenatal ultrasound has enabled accurate diagnosis. Materials and Methods: The authors report a 26-year-old patient, gravida 4 para 1, who was referred to this hospital, carrying a fetus with immature mediastinal teratoma. Results: At 27 weeks of gestation, a routine prenatal ultrasound suggested the fetus had a mass at the anterior mediastinum, accompanied by pulmonary hypoplasia, pleural and peritoneal effusion, subcutaneous edema of head and chest, and polyhydramnios. After the therapeutic abortion, the gross anatomy confirmed the mediastinal mass. The histological examination showed that the mass was a grade 2 immature teratoma. Conclusions: The mother of the fetus had been exposed to plaster, paint, and paint-thinner in the first trimester of pregnancy, suggesting that these chemical contacts may be one of the causes of the disorder.