The population-based Augsburg Coronary Event Register (330,000 residents, age 25-74 years) has registered a total of 1012 cases of acute myocardial infarction (AMI) in 1985 and 1021 AMI in 1986 and categorized them on the basis of the current WHO diagnostic algorithm for AMI. The register is designed for longitudinal comparisons of annual AMI risk (incidence, attack rate, death rate), and the risk to the AMI patients themselves (28-day case fatality). The methodology and specific issues encountered during registration and data evaluation are described. With an estimated 95% completeness of case finding, the quality control data review which the register conducts annually shows a consistency of specific data structures which indicate stable case finding and validation procedures. However, local conditions which affect case finding and data completeness per case are responsible for the creation of subsets of AMI which are in turn distinguished by differences in diagnostic category structures. With regard to the study objectives, the differences among subsets appear to have the least effect on rate calculations if DEFINITE and POSSIBLE AMI are combined. The implications of methodological variations and subset differences within and across registers on annual rate calculations and result comparisons are discussed.