Background: The incidence of acute myocardial infarction has declined in several Western countries during the last decades. The incidence and mortality of acute myocardial infarction follow a seasonal pattern. We examined if changes in the incidence of acute myocardial infarction were associated with any changes in seasonality. Methods: The study was based on 17,989 patients hospitalized with first-time acute myocardial infarction identified in the Hospital Discharge Registry of the County of North Jutland, Denmark, from 1 January 1983 to 31 December 1999. The seasonality of acute myocardial infarction was estimated using a Poisson regression model. Results: The incidence rate decreased by 3.2%(95%confidence interval: 2.7-3.3%) annually. Hospitalizations followed different seasonal patterns depending on age, but not on gender. In the < 59-year-old group, the seasonal pattern was dominated by a broad spring peak(April/May) and a minor autumn peak. With increasing age, the spring peak decreased while the autumn peak increased and moved towards December. A seasonal pattern dominated by one peak(December) and one trough(August) was found in the ≥80-year-old category. The shape and extent of these seasonal patterns remained stable throughout the study period despite the decline in hospitalizations for acute myocardial infarction. Conclusion: Hospitalizations for first-time acute myocardial infarction decreased from 1983 to 1999, but the seasonal pattern remained stable over time. Background: The incidence of acute myocardial infarction has declined in several Western countries during the last decades. The incidence and mortality of acute myocardial infarction follow a seasonal pattern. We examined if changes in the incidence of acute myocardial infarction were associated with any changes in seasonality . Methods: The study was based on 17,989 patients hospitalized with first-time acute myocardial infarction identified in the Hospital Discharge Registry of the County of North Jutland, Denmark, from January 1983 to 31 December 1999. The seasonality of acute myocardial infarction was estimated Results: The incidence rate decreased by 3.2% (95% confidence interval: 2.7-3.3%) annually annually. Hospitalizations followed different seasonal patterns depending on age, but not on gender. In the <59-year-old group, the seasonal pattern was dominated by a broad spring peak (April / May) and a minor autumn peak. With increasing age, the spring peak de creased while the autumn peak increased and moved towards December. A seasonal pattern dominated by one peak (December) and one trough (August) was found in the ≥80-year-old category. The shape and extent of these seasonal patterns were stable during the study period despite the decline in hospitalizations for acute myocardial infarction. Conclusion: Hospitalizations for first-time acute myocardial infarction decreased from 1983 to 1999, but the seasonal pattern remained stable over time.