Because acute myocardial infarction causes significant morbidity and mortality, a correct diagnosis in the accident and emergency department is important so that early treatment including thrombolytic therapy can be given. The aim of this study was to evaluate the reasons for missed diagnosis of acute myocardial infarction in the accident and emergency department, and the implications. All patients admitted to our coronary care unit in 1995 with the confirmed diagnosis of acute myocardial infarction were analysed retrospectively. The demographic data, clinical profiles, diagnosis made at the accident and emergency department and feasibility of thrombolytic therapy were assessed. Analysis of the electrocardiograph by the accident and emergency department doctor and the coronary care unit doctor were also compared. Forty-three out of 159 patients (27.0%) with acute myocardial infarction were missed in the accident and emergency department. The diagnoses made were mostly angina or chest pain. Absence of chest pain (25.6%) [vs. 10.2% in correct diagnosis group, p < 0.05] and lack of ST elevation in electrocardiograph (62.8%) [vs. 18.1% in correct diagnosis group, p < 0.0001] were the main predisposing factors for missed diagnosis. Because of missed diagnosis, only 25.6% (vs. 67.2% in correct diagnosis group, p < 0.01) of patients were admitted to the coronary care unit. About one-third (34.9%) of missed diagnosis patients (vs. 6.0% in correct diagnosis, p < 0.01) did not receive thrombolytic therapy because of delayed diagnosis. In the missed diagnosis group, 34.8% of them might be avoidable, if electrocardiogram interpretation was more accurate. More education and training of the involved medical personnel might improve the overall situation.