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Patients with thymoma may have a predisposition toward extrathymic neoplasia. To understand the lifetime risk and incidence of extrathymic neoplasia in patients with thymoma, we evaluated extrathymic neoplasms diagnosed either before or after the diagnosis of thymoma.We queried the Surveillance, Epidemiology, and End Results (SEER) cancer database and identified patients with thymoma and extrathymic neoplasms. We collected demographic and treatment data, calculated the incidence of each extrathymic neoplasm (adjusting for age), and compared the incidence of extrathymic neoplasm in patients with thymoma with the age-adjusted incidence in the SEER database general population.Of 2,171 patients with thymoma in the SEER database, 306 (14.1%) had extrathymic primary cancers. Extrathymic neoplasms were diagnosed before the diagnosis of thymoma in 88 patients and after the diagnosis of thymoma in 206 patients. In 12 patients, separate extrathymic neoplasms were diagnosed both before and after thymoma diagnosis. The incidence of extrathymic cancers in patients with thymoma (8,224 per 100,000 persons) was significantly higher than in the SEER general population (459 per 100,000 persons; p < 0.001). The standardized incidence ratio for extrathymic cancer was also significantly increased, in particular for lymphomas, leukemias, esophageal cancer, and lung cancer. Increased age at diagnosis (p < 0.001), longer survival after diagnosis (p < 0.001), and male sex (p = 0.041)—but not radiation therapy or surgery—were significant risk factors for the development of extrathymic cancers in patients with thymoma.The incidence of extrathymic neoplasia is significantly higher in patients with thymoma than in the general population and occurs both before and after the diagnosis of thymoma.