Ten cases of spindle cell thymomas with prominent papillary and pseudopapillary features were presented. The patients were 7 men and 3 women between the ages of 47 and 75 years. Clinically, 3 patients were asymptomatic, 1 patient presented with chest pain, 4 patients with shortness of breath, and 1 patient with a history of pulmonary tuberculosis. One case was found during an autopsy procedure. Nine patients underwent complete surgical resection of their mediastinal tumors, which varied in size from 4 to 9 cm in greatest diameter. Histologically, all tumors showed a spindle cell appearance (WHO type A) with elongated nuclei and inconspicuous nucleoli. Scattered lymphocytes were present admixed with the spindle cellular proliferation. In addition, all tumors showed prominent areas of papillary and pseudopapillary features, which varied in size and type. In some cases, prominent areas of hyalinization were also present, whereas in other cases the papillary-like changes were composed of edematous projections, which imparted these tumors a unique morphologic growth pattern. Three tumors were encapsulated, whereas 7 other tumors were invasive. Immunohistochemical studies for keratin CAM5.2 and keratin 5/6 showed strong positive reaction, whereas other stains including CEA, calretinin, CD-31, and thyroglobulin were negative. Follow-up information showed that 2 patients are alive and well, whereas 3 patients have died. No follow-up information was obtained in 4 patients. The current morphologic appearance has not been previously emphasized in thymomas, which is important to recognize to avoid misdiagnosis with other mediastinal neoplasms.