The acute myocardial infarction (AMI) register of the FINMONICA study, the Finnish part of the WHO-coordinated multinational MONICA project, operates in the provinces of North Karelia and Kuopio in eastern Finland and in Turku, Loimaa and in communities around Loimaa in southwestern Finland. The AMI register serves as an instrument for the assessment of trends in mortality from coronary heart disease (CHD) and of the incidence and attack rates of AMI among 25-64-year-old residents of the study areas. This report describes the methods used in the FINMONICA AMI register and the findings during the first 3 years of the study, in 1983-1985. The criteria of the multinational WHO MONICA project were used in the classification of fatal events and in the diagnosis of non-fatal definite AMI, but based on the experience within the FINMONICA study, stricter diagnostic criteria than those originally described in the WHO MONICA protocol were used for non-fatal possible AMI. This led to a marked improvement in the comparability of the data from the three study areas with regard to the incidence and attack rates of non-fatal AMI. During the 3-year period the total number of registered events was 6266 among men and 2092 among women. Among men the incidence and attack rates of AMI and mortality from CHD were higher in eastern than in southwestern Finland. Also among women the incidence and attack rates of AMI were higher in eastern than in southwestern Finland, whereas there was no regional difference in mortality from CHD among women. The mortality findings of the FINMONICA AMI Register were in good agreement with the official CHD mortality statistics of Finland.