The Hunter Valley Heart Attack Study was conducted throughout 1979 to investigate all cases of suspected heart attack among permanent residents 20 to 69 years of age in a well-defined area in New South Wales, Australia. The study design followed that for the Myocardial Infarction Community Registers coordinated by the World Health Organization (WHO) in Europe and elsewhere around 1971. The attack rates obtained confirm that in Australia myocardial infarction incidence, like mortality, is high by international standards. Case fatality rates were somewhat lower than those reported elsewhere. To discover whether heart attack rates are changing with the decrease in heart disease mortality, it is proposed to carry out longitudinal surveillance in the study population. Experience with this baseline survey indicates that there may be difficulties in reliably interpreting the WHO diagnostic criteria and that changes in medical management of heart disease may affect the numbers of cases diagnosed. This highlights an epidemiologic need for clear definitions of myocardial infarction.