Thymoma is a rare malignancy of unknown etiology. Based on cancer registry data, the overall incidence of thymoma in the U.S. is 0.13 per 100,000 person-years. Thymoma is exceedingly uncommon in children and young adults, rises in incidence in middle age, and peaks in the seventh decade of life. For unknown reasons, thymoma incidence is especially high among Asians and Pacific Islanders in the U.S. While several studies based at single treatment centers have suggested that thymoma patients have a broadly increased risk for other malignancies, follow up data from U.S. cancer registries support a more limited spectrum of cancer risk. In particular, people with thymoma have a subsequently elevated risk for developing B-cell non-Hodgkin lymphoma, consistent with an effect of immune disturbance arising from the thymoma or its treatment. Based on limited data, thymoma patients may also have an elevated risk for developing soft tissue sarcomas. While these descriptive epidemiologic data may yield clues to the etiology of thymoma, large multi-center case-control studies will be required to formally evaluate environmental and genetic risk factors.