Aims To examine the general influence of the definition of fatal and non-fatal acute myocardial infarction and coronary deaths on the estimation of in-hospital case- fatality, and to show how the definition of acute myocardial infarction influences time-trends of hospital mortality over 11 years. Methods and Results As part of the World Health Organization’s MONICA (multinational Monitoring of Trends and Determinants in Cardiovascular Disease) Project in Augsburg all patients aged 25–74 years with a suspected diagnosis of acute myocardial infarction who were hospitalized in the study region’s major clinic were registered prospectively between 1985 to 1995 (n = 4889). Patient information, including short-term survival status, was obtained from medical records, by interview of surviv- ing patients, and municipal death certificate files which were validated by an extended identification and validation pro- cess. In-hospital case fatality was estimated according to di ff erent definitions which closely followed the international MONICA criteria. Epidemiological definitions comprised definite and possible acute myocardial infarction, and events with unclassifiable deaths, while the clinical defi- nition was restricted to definite infarction. Overall, case fatality by the epidemiological definitions was 28 to 29·8% (23·5% of those treated in a coronary care unit) compared to 13·5% using the clinical definition. While over the 11 years, the reduction in case fatality according to the epi- demiological definitions was modest, highly significant decreases were observed by applying the clinical definition (from 15·8% in 1985–1988 to 10·8% in 1993–1995, P <0·001 adjusted for age and sex). The discrepancy in case fatality between the definitions is explained by the high proportion of patients who die very early (about 70% of all fatal events during the first 24 h) with the consequence of missing data which may preclude a definite diagnosis of acute myocardial infarction. Conclusions Applying a broader definition of acute myo- cardial infarction reveals that in-hospital mortality is higher than believed until now, and it implies that our e ff orts must be intensified to reduce overall in-hospital coronary heart disease mortality.