Trends in the incidence of and mortality from myocardial infarction in Perth, Western Australia, were studied for the period 1971 to 1982. The estimated age-adjusted incidence rate of myocardial infarction fell by 24% in males and 37% in females over this period. The rate fell in each of six age-sex groups, but the proportional decrease in those aged less than 45 years was nearly double that in those aged 45 years and over, as it was in females compared with males and in fatal cases compared with nonfatal cases. Over 80% of the decline in mortality was in deaths outside hospital. This and seven other comparable studies all showed a net decrease in the estimated incidence of myocardial infarction in both sexes, with median annual decreases of 2% in males and 3% in females. It is concluded that the decline in the incidence of myocardial infarction contributed substantially to the observed decrease in ischemic heart disease mortality, and that the most plausible explanation for the decline in incidence is a reduction in the prevalence of risk factors for myocardial infarction.