A case of immature mediastinal teratoma in a 43-year-old Japanese man is reported. The tumor, which was multicystic with solid zones and measured 12 x 6 x 8 cm, arose in the anterior mediastinum. The serum alpha-fetoprotein (AFP) level was elevated to 5,114 ng/ml before surgery. Histologically, the solid zones showed an admixture of irregular glands lined by columnar or cuboidal epithelium set in a spindle cell stroma, some foci of primitive neural tissue, and scattered small nests of hepatoid cells. Immunohistochemically, the hepatoid cells and epithelia lining some of the cysts showed a strongly positive reaction for AFP. Eight months after surgery, the patient died of respiratory failure caused by a rapidly growing massive recurrent tumor, which measured 40 x 24 x 13 cm, in the left thoracic cavity. However, the elevated serum AFP level had been decreasing during the course of the recurrence in response to chemotherapy. The recurrent tumor showed remarkable proliferation of loose mesenchymal tissue without primitive neural tissue. These findings suggest that immature mediastinal teratoma in adults is highly malignant, and that non-AFP-producing mesenchymal tissue played a critical role in forming the rapidly growing massive recurrent tumor in the present case.