In the Who Monica Project mortality and morbidity from acute myocardial infarction is being monitored in 39 Collaborating Centres in 26 countries for a period of 10 years. The myocardial infarction registration procedures have been standardized and suspect coronary events are classified into diagnostic categories according to common criteria. This paper presents ischaemic heart disease mortality figures, based on routine statistics for the study populations and discusses cross-sectional comparability of morbidity data from event registration. Because of differences in availability of data used for diagnostic classification of events (such as autopsy data) the proportions of different diagnostic categories vary from centre to centre. There are, therefore, problems in cross-sectional comparison of morbidity data between the centres. The primary target of event registration in the project is to monitor morbidity changes within centres during the ten-year period. This goal should not be affected by the problems described in this paper.