Aims: Contemporary data on the incidence of unrecognized myocardial infarction(MI) among subjects aged 55 and older are limited. Methods and results: We studied the incidence of recognized and unrecognized MI in the Rotterdam Study, a population-based cohort of men and women aged 55 and older. The baseline examination was performed during 1990-93, with follow-up examinations during 1994- 95, and 1997-2000. Baseline and follow-up 12-lead ECGs were analysed by the Modular ECG Analysis System. The 5148 participants who had no evidence of prevalent infarction were the subjects for analysis. Incident recognized infarction was defined as the occurrence of a fatal or non-fatal event coded as 121 according to the International Classification of Diseases, 10th edition. A repeat ECG was available in 4187 subjects. An unrecognized infarction was considered to have occurred if there was electrocardiographic evidence in the absence of a clinically recognized event. During a median follow-up of 6.4 years, 141 incident recognized infarctions occurred and the incidence rate of this event was 5.0 per 1000 person years. The incidence was higher in men(8.4) than in women(3.1). The incidence rate of unrecognized infarction was 3.8 per 1000 person years. Men(4.2) and women(3.6) had approximately similar incidence. Hence, the proportion of unrecognized infarction was lower in men(33% ) than in women(54% ). This difference in proportion of unrecognized infarctions was independent of age. Conclusion: A high proportion of incident MIs remains clinically unrecognized. As a history of MI is associated with an increased risk of repeat cardiovascular complications, our data suggest a need for periodical electrocardiographic screening to recognize(prevalent) infarctions and to install effective preventive treatment in those aged 55 and older. Methods and results: We studied the incidence of recognized and unrecognized MI in the Rotterdam Study, a population-based cohort of men and women The baseline examination was performed during 1990-93, with follow-up examinations during 1994-95, and 1997-2000. Baseline and follow-up 12-lead ECGs were analyzed by the Modular ECG Analysis System. The 5148 participants who had no evidence of prevalent infarction were the subjects for analysis. Incident recognized infarction was defined as the occurrence of a fatal or non-fatal event coded as 121 according to the International Classification of Diseases, 10th edition. A repeat ECG was available in 4187 subjects. An unrecognized infarction was considered to have occurred if if there was electrocardiographic evidence in the absence of clinically recognized event. During a median follow-up of 6.4 years, 141 incidences of unrecognized infarction was 3.8 per 1000 person years. The incidence was higher than men (8.4) than in women (3.1). This difference in proportion of unrecognized infarctions was independent of age. Conclusion (1) Men (4.2) and women (3.6) had less similar incidence. : A high proportion of incident MIs remains clinically unrecognized. As a history of MI is associated with an increased risk of repeat cardiovascular complications, our data suggest a need for periodical electrocardiographic screening to recognize (prevalent) infarctions and to install effective preventive treatment of those aged 55 and older.