Twenty-five cases of thymoma with prominent cystic and hemorrhagic changes and areas of necrosis and infarction are presented. The patients were 11 women and 14 men between the ages of 18 and 73 years (median 45.5 years). Clinically, nine patients were asymptomatic and their mediastinal tumor was discovered on routine chest radiograph. Sixteen patients presented with symptoms of chest pain and cough. All patients underwent surgical resection of their tumor. Grossly, the tumors were described as well circumscribed and encapsulated, with the exception of two that showed infiltration of pleura and pericardium. The tumors measured from 4 to 13 cm in greatest dimension. On cut surface they showed prominent cystic areas and foci of hemorrhage and necrosis. Histologically, the tumors contained solid areas showing an admixture of round to oval epithelial cells devoid of atypia admixed with small lymphocytes in varying proportions. Cystic changes with areas of necrosis, infarction, and hemorrhage were present in all cases and comprised extensive areas of the tumors. The areas of infarction showed features of ischemic necrosis and were always intimately associated with vaso-occlusive and thrombotic phenomena and with cystic and hyperplastic changes of adjacent thymic epithelium. Clinical follow-up in 14 patients showed that 11 were alive and well from 1 to 18 years after surgery (median follow-up 9 years). Three patients died: one of complications during the immediate postoperative period, one because of colonic adenocarcinoma 9 years after diagnosis of the mediastinal tumor, and one because of pneumonia 6 years later. The two patients with invasive tumors were lost to follow-up. The present study appears to indicate that areas of hemorrhage and necrosis in well encapsulated, noninvasive thymomas do not portend an adverse prognosis.