The epidemiology of primary immune thrombocytopenia (ITP) is not well-characterized in the general population. Most published studies, which have included relatively small numbers of ITP patients, have been conducted in England or Scandinavian countries. No epidemiologic data from Asian countries have been published. This study describes the epidemiology of ITP in a Japanese population. We analyzed the database registry of the Ministry of Health, Labour, and Welfare of Japan, and extracted newly diagnosed acute and chronic ITP patients with a platelet count of <100 × 10(9)/L. From 2004 to 2007, 7,774 cases of ITP were reported, giving an overall incidence of 2.16/100,000/year. The incidence differed greatly between males and females, being 1.72 and 2.58, respectively. The median age of the total affected population was 56 years old. In male patients, there was a striking preponderance of boys below 4 years and a very high peak among those aged 75-89 years. In female patients, the number of ITP patients appeared to show a trimodal distribution by age, with the first peak representing patients below 4 years, the second peak those aged 20-34 years, and the third peak those aged 50-89 years. In conclusion, the incidence of ITP in Japan is not markedly different from that of European countries studied to date. This population-based study reveals that, contrary to previously published studies, the maximum age-specific incidence is in the eighth decade.