In an attempt to characterise the clinical features, incidence and outcome of essential thrombocythaemia (ET) we report our experience in a large unselected series of patients from a well defined region. All new cases of ET in the County of Copenhagen were registered during the period 1977-98. We identified 96 cases of ET, yielding an age-and sex-adjusted annual incidence rate of 0.59/100.000 and a point-prevalence at last follow up of 11/100.000. The overall incidence rate was 0.31 and 1.00 per 100.000 population during the consecutive periods 1977-89 and 1990-98, respectively, corresponding to a 3.2-fold increase. Median age at diagnosis was 67 yr (females 68 yr, males 66 yr, range 18-87 yr), and the female to male (F/M) ratio was 2.6:1. At diagnosis, 52% of the patients displayed no ET-related symptoms and were discovered fortuitously by a routine platelet count. Forty-eight percent presented with thrombohaemorrhagic phenomena, of which microvascular disturbances of the central nervous system (CNS), extremities and skin were most frequently observed (23%). Compared to patients diagnosed after 1989, patients diagnosed before 1990 had a significantly higher mean platelet count, white blood cell (WBC) count, lactate dehydrogenase (LDH) value and alkaline phosphatase value. With a median follow up of 70 months, 5-yr survival was 76%, significantly lower than the expected survival of an age- and sex-matched control group (p = 0.0052). Thirty-seven patients experienced a total of 55 thrombohaemorrhagic events during follow-up, corresponding to an incidence of thrombosis and microvascular disturbances or haemorrhage of 8.1% per pt-yr and 2.5% per pt-yr, respectively. The number of patients experiencing thrombosis or microvascular disturbances was significantly higher among the 29 patients who never received acetylsalicylic acid (ASA) compared to the 67 patients who received ASA during follow up (45% vs. 21%; p =0.017). This study provides population-based data suggesting the benefit of treatment with low-dose ASA in a non-selected population of patients with ET.