slowly progressing anterior mediastinal teratoma. This is an extremely rare case, not only in terms of the presence of this type of tumor in an elderly patient but also in terms of its evolution to malignancy. This, together with the fact that, in the literature, we have found only three studies of carcinoid tumors arising within mediastinal teratomas, has motivated us to write this article. Case reportA 66-year-old male patient, born in the city of São Luiz (in the state of Maranhão) and a resident of the city of São Paulo (in the state of São Paulo), was admitted to IntroductionTeratomas are germ cell tumors originating from the three embryonic germ layers. These tumors can be found at multiple sites, and most are benign. They are the second most common tumors of the anterior mediastinum, after thymomas, and are most often seen in young adults.(1) They account for 8 to 13% of all tumors occurring in this region. If they are treated surgically, the prognosis is quite favorable. Although there is little data in the literature regarding the evolution of a teratoma to malignancy, it is known to be an extremely rare occurrence. (2,3) We report the case of a 66-year-old patient submitted to resection of a carcinoid tumor (revealed by anatomopathological examination) that had arisen within a AbstractHere, we report the case of a patient with a slowly-progressing anterior mediastinal teratoma submitted to surgical resection. The anatomopathological examination of the sample revealed malignant degeneration to carcinoid tumor. Such evolution is very rare, and we have found only three related studies in the literature. We describe the clinicopathological features of the tumor and discuss the treatment administered. The evolution was satisfactory, and the patient was submitted to oncological treatment.Keywords: Mediastinal neoplasms; Teratoma; Carcinoid tumor; Mediastinum/surgery. ResumoNo presente artigo, relatamos o caso de um paciente portador de teratoma de mediastino anterior, de evolução lenta, o qual foi submetido à ressecção cirúrgica. O exame anatomopatológico da peça revelou degeneração maligna para tumor carcinóide. Tal evolução é extremamente rara, sendo encontrados na literatura apenas três artigos correlatos. Apresentamos uma descrição clínico-patológica do tumor e, por fim, discutimos a conduta terapêutica. Houve evolução satisfatória, e o paciente foi submetido a tratamento oncológico.Descritores: Neoplasias do mediastino; Teratoma; Tumor carcinóide; Mediastino/cirurgia.