OBJECTIVES: Immune thrombocytopenic purpura (ITP) is a rare and life-threatening autoimmune disorder characterized by increased destruction and impaired production of blood platelets by auto-antibodies. International epidemiological studies reported yearly incidence and prevalence of respectively 2 and 10 cases/100,000 persons. No Belgian epidemiological data are currently available. To estimate the incidence and prevalence of adult chronic ITP in Belgium using hospital records. METHODS: The incidence of adult chronic ITP was estimated using the National statistics on Diagnosis Related Group for splenectomy (DRG 650, year 2004). The prevalence was estimated using the longitudinal IMS Hospital Disease Database (year 2006), including data of 34.3% of Belgian hospitals. All admissions to hospital with a diagnosis of Primary Thrombocytopenia (ICD-9-CM 287.3) were retrieved for patients aged >20 years. The repartition of cases by age and sex was investigated. RESULTS: A total of 229 splenectomies were performed in adults in 2004. Assuming that 10-30% of splenectomies are ITP-related and 20-40% of chronic ITP patients are splenectomized, this results in a yearly incidence of 0.7-4.3 new ITP cases per 100,000 adults, i.e. 57-344 new cases/year. In 2006, 657 admissions for primary thrombocytopenia were recorded in the Hospital Disease database, for 350 distinct adult patients. This corresponds to an estimated prevalence of 12.6 per 100,000 adults per annum, i.e. 1020 patients. A quadratic increase of the prevalence was observed with age: from 5.8 to 39.8 cases per 100,000 per year in the age groups of 20-29 years to ≥75 years. The prevalence was 2.7 times higher in women aged 20-55 years vs. men (p<0.001), but similar in patients aged older. CONCLUSIONS: Although non-hospitalized patients were not captured in this analysis, these Belgian incidence and prevalence data are consistent with published international data. The prevalence of ITP increases substantially with age. Prevalence is also higher in women in younger age groups.