BACKGROUND- The incidence of myocardial infarction has declined during the past 4 decades in many populations. However, there are limited population data measuring trends in acute coronary syndromes (ACS). We therefore examined temporal trends in the incidence of hospitalized ACS by age and sex in a population-based cohort. METHODS AND RESULTS- The Western Australian Data Linkage System, a repository of linked administrative health data, was used to identify 29 421 incident ACS hospitalizations between 1996 and 2007. Poisson log-linear regression models were used to calculate incidence rate changes. Age-standardized incidence rates of ACS declined annually in men by 1.7% (95% confidence interval [CI], -2.1 to -1.3) and in women by 1.6% (95% CI, -2.1 to -1.0). These declining rates were underpinned by annual reductions in the incidence of unstable angina (men, -3.0%; 95% CI, -3.7 to -2.4; women, -2.5; 95% CI, -3.3 to -1.7), whereas annual changes in myocardial infarction incidence were less (men, -1.0%; 95% CI, -1.5 to -0.5; women, -0.8%; 95% CI, -1.6 to 0). However, the overall trends masked age group differences, with ACS incidence increasing annually in 35- to 54-year-old women (2.3%; 95% CI, 1.0 to 3.8), predominantly driven by increasing incidence of myocardial infarction. CONCLUSIONS- The age-standardized incidence of ACS decreased significantly in Western Australia from 1996 to 2007. However, an increase in ACS incidence in women ages 35 to 54 years is troubling and warrants further investigation.