BACKGROUND Elevated heart rate (HR) has been found to be related to an increased death rate in patients with acute myocardial infarction (AMI), but sex differences and optimal timing for HR measurement have not been sufficiently investigated. OBJECTIVE To verify the predictive value of HR for one-year mortality in a cohort of subjects hospitalized for AMI, with men and women considered separately. METHODS HR was measured in 424 patients (303 men and 121 women) with constant sinus HR, on the first, third and seventh days after hospital admission for AMI. Clinical and laboratory data were obtained on the same days. All patients were followed up for one year. RESULTS Among the men, the one-year mortality rate was 5% for the subjects with a seven-day HR of less than 80 beats/min, and the one-year mortality rate was 39% for patients with a seven- day HR of 80 beats/min or more (P<0.0001). Among the women, the differences in mortality related to HR were not significant. In a multivariate Cox regression analysis, the relative risks of mortality in men who had an HR of 80 beats/min or more were 3.1 (CI=1.4 to 7.0, P=0.003) on the first day, 4.1 (CI=1.8 to 9.8, P=0.001) on the third day and 8.6 (CI=2.9 to 27.0, P<0.0001) on the seventh day. In the 203 men in whom echocardiographic left ventricular ejection fraction was measured, an interactive effect of high HR with depressed ejection fraction on mortality was found. Beta-blocking therapy influenced HR during AMI but did not influence the HR-mortality association. CONCLUSIONS The results of the present prospective study show that HR measured during the first week after admission for AMI is an important predictor of mortality in men. The predictive power of HR increased from the first to the seventh day after AMI.