AIMS: To analyse the effect of the change in diagnostic criteria for acute myocardial infarction (AMI) and the use of troponin as a diagnostic marker on the hospitalization rate and mortality of hospitalized AMI patients from 1994 to 2001. METHODS AND RESULTS: Patients (> or =30 years) admitted for their first AMI were identified using the National Patient Registry in Denmark. We registered when each hospital introduced troponin as a diagnostic marker. The reported hospitalization rate decreased until 1998 and then increased substantially from 1999 to 2001 from 3472 to 4163 per million inhabitants (19.9%) for men and from 1648 to 2020 per million inhabitants (22.6%) for women. Troponin use was associated with a significant 14% increase in hospitalization rate in this period [rate ratio 1.14, 95% confidence interval (CI) 1.11-1.18]. The effect of troponin was greatest among patients 70 years and older (rate ratio 1.19, 95% CI 1.14-1.23). The 28 day mortality decreased steadily from 25.9% in 1994 to 17.5% in 2002 (32.4%) and was not affected by troponin use. CONCLUSION: The reported hospitalization rate for AMI increased significantly after the new diagnostic criteria for AMI were introduced. The measurement of cardiac troponins further increased the hospitalization rate. The mortality among hospitalized patients with AMI declined steadily and was not affected by the use of troponins.