We describe 250 cases of thymoma with emphasis on World Health Organization (WHO) histologic subtyping. The patients were 120 males and 130 females between the ages of 13 and 92 years. Surgical resection was performed, and histologic material was evaluated in every case. Macroscopically, the tumors varied in size from 3 to 20 cm in greatest diameter; about 12% were encapsulated, and about 88% were invasive tumors. A minimum of 5 sections of tumor was evaluated. Histologically, following the schema proposed by the WHO, 21.6% of thymomas were type A, 1.23% type B1, 3.2% type B2, and 9.2% type B3. More than 50% of tumors after subtyping fell into the mixed categories, which, in essence, diminishes the clinical impact of histologic subtyping over staging. The study herein described highlights that all thymomas had the potential to become invasive tumors.