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The cause of thymoma is unknown. No population‐based study has described demographic patterns of thymoma incidence. Previous reports have linked thymoma with diverse subsequent malignancies, but these associations are uncertain. We used Surveillance, Epidemiology and End Results (SEER) data to study the incidence of malignant thymoma by sex, age and race in the United States (1973–1998). Incidence was modeled with joinpoint regression (for age) and Poisson regression. We also used SEER data to compare malignancies following thymoma diagnosis with those expected from general population rates, calculating the standardized incidence ratio (SIR, observed/expected cases) to measure risk. The overall incidence of malignant thymoma was 0.15 per 100,000 person‐years (849 cases). Thymoma incidence increased into the 8th decade of age and then decreased. Incidence was higher in …