Studies have reported a 1·3- to 2·2-fold higher mortality rate among patients with primary immune thrombocytopenia (ITP) compared to the general population. However, long-term mortality estimates as well as cause-specific mortality data are sparse. In our population-based cohort of adult patients with newly diagnosed ITP and up to 37 years of follow-up, the 5-year, 10-year and 20-year mortality among the ITP patients was 22%, 34% and 49%, respectively. The mortality in the ITP cohort was consistently higher than in the in the general population cohort yielding an adjusted hazard ratio (HR) of 1·5 [95% confidence interval (CI): 1·2-1·8]. The adjusted HRs of mortality due to cardiovascular disease, infection, bleeding and haematological cancer were 1·5 (95% CI: 1·1-1·5), 2·4 (95% CI: 1·0-5·7), 6·2 (95% CI: 2·8-13·5) and 5·7 (95% CI: 2·1-15·7), respectively, whereas mortality due to solid cancer and other causes were similar in ITP patients and the general population. We conclude that mortality rates among ITP patients are higher than in the general population, predominantly as a result of increased cardiovascular disease, infection, bleeding and haematological cancer cause-specific mortalities.