A 43-year-old female who had dyspnea and had been diagnosed as myasthenia gravis was found to have a mass lesion of the anterior mediastinum that protruded to the left and contained calcified areas on radiological investigation. The patient was operated on with a preliminary diagnosis of thymoma. Macroscopically the tumoral lesion was 5x4.5 cm in size and was light tan, firm and solid without necrosis or hemorrhage. Histological investigation showed epithelial islands without atypical features scattered within large areas of hyalinized collagenous tissue and the presence of immature T lymphocytes some of which were TdT positive, which led to a preliminary diagnosis of sclerosing thymoma. Sclerosing thymoma is a rare type of thymoma and has first been reported in 1994. Since there are only a few reports in the literature detailed information on clinical features and pathogenesis is needed. The possibility of sclerosing thymoma should always be kept in mind in the differential diagnosis of mediastinal lesions with marked sclerosis that can lead diagnostic difficulties especially when evaluating small mediastinoscopic biopsies.