Before looking at the trends in coronary event rates and case fatality rates in the WHO MONICA Project, it is necessary to assess the consistency and validity of the data. In the Newcastle MONICA Collaborating Centre, two methods have been used. One involves monitoring data quality by comparisons with external data systems such as hospital discharge data and official mortality records. The other is to examine the internal consistency of MONICA diagnostic findings. For fatal myocardial infarction (MI) or coronary death, there is consistent evidence of a decline, but the MONICA data are not adequate to assess relative changes in sudden compared to non-sudden death rates. For non-fatal definite MI there was an increase early in the study, possibly due to a change in methods, but rates have now stabilized. For non-fatal possible MI there has been a steady increase in rates for events which may be becoming less severe. This is consistent with increasing hospital admissions for subacute ischaemic heart disease (IHD) and angina. This paper exemplifies the importance of maintaining internal and external surveillance of the quality of the data.