A rare case of malignant intrapulmonary teratoma is reported. The patient, 50 year old female, had suffered from symptoms characteristic of a benign intrapulmonary teratoma more than twenty years. After the confirmation on a bronchoscopic biopsy of adenocarcinoma arising in the pre-existing benign teratoma, left pneumonectomy was performed. The adenocarcinomatous component metastasized to the contralateral lung one year after the pneumonectomy, and the patient died in 3 months after the metastasis. The histogenesis, clinical behavior and pathologic features of intrapulmonary malignant teratoma are discussed, and the rarity of the pulmonary location of this lesion is emphasized.