Summary. The true incidence and prognosis of autoimmune thrombocytopenic purpura (ITP) in adults is unknown. We present the results of a prospective study in a population-based cohort of newly presenting adults (≥ 16 years) with ITP and platelet count of < 50 × 109/l, which took place between 1 January 1993 and 31 December 1999 in the former Northern Health Region in the UK (population 3·08 million). A total of 245 cases were confirmed by bone marrow examination with a median follow-up of 60 months (range 6–78 months). There were 134 females/111 males (1·2:1). Overall incidence was 1·6 per 105 per annum. Absolute incidence was similar for both sexes, with highest age-specific incidence in those aged > 60 years. Thirty patients (12%) presented with frank bleeding, and 28% were asymptomatic. Forty-five patients (18%) received no treatment, and 135 (55%) received first-line treatment only. Thirty patients (12%) underwent splenectomy. There were four deaths (1·6%) from bleeding and/or the complications of therapy in this cohort, but only one was in the acute phase of the illness. The majority of patients (155 out of 245) achieved remission (platelet count > 100 × 109/l), with a further 59 (24%) in partial remission with no symptoms (platelet count 30–100 × 109/l). This population-based study suggests that the traditional view of adult ITP as being a predominantly chronic disease that preferentially affects females needs to be modified.