The recent decline in coronary heart disease (CHD) mortality has been attributed to reduction in risk factors, improved management and the possibility of statistical artifacts. The purpose of this study is to assess the impact of geographic and time variation in the coding of cause of death from death certificates into ICD codes upon CHD mortality rates in Canada. Equal samples of death certificates were recoded for Nova Scotia and Saskatchewan for each of the years 1970 and 1984: 1) a first set of 1,600 death certificates originally coded as acute myocardial infarction (AMI) and 2) a second set of 800 death certificates from all causes of death. The coding error rates increased with age and with the number of contributing and underlying causes of death reported on the death certificates. The net effect of false positive and false negative AMI codes on death certificates did not vary significantly by province or year. Thus, variation of death certificate coding over time and geographic regions do not contribute toward the explanation of the AMI mortality rate decline.