We present 10 cases of "ancient" (sclerosing) thymomas in 4 women and 6 men (age range, 34-73 years; mean, 53.5 years). Clinically, 4 patients had a history of shortness of breath and chest pain of several weeks' duration, 1 had a history of myasthenia gravis, and 5 were asymptomatic (anterior mediastinal masses discovered on routine chest radiographs). All underwent complete surgical resection of the anterior mediastinal mass. The tumors were light tan and solid, without areas of hemorrhage or necrosis, and 5 to 10 cm in greatest diameter. Extensive areas of hyalinized fibroconnective tissue constituting about 85% to 90% of the tumor mass was the main tumor feature. Focal areas of conventional thymoma also were present in all tumors. Follow-up information was obtained for 8 patients. Of these 8 patients, 6 died, all of unrelated causes (congestive heart failure, renal insufficiency, and pulmonary edema), and 2 were alive 1 and 6 years after surgical resection. The present cases highlight an important histopathologic feature of thymomas, which can pose problems in diagnosis, namely when dealing with small mediastinoscopic biopsy specimens.