It is still unclear whether the presence of dementia has a negative effect on survival in elderly patients with acute myocardial infarction (AMI). Therefore, using data from the Tokai Acute Myocardial Infarction Study II (TAMIS-II), we set out to clarify the differences in in-hospital and long-term mortality between AMI patients with and without dementia. The study was a prospective study of all consecutive patients admitted to 15 acute care hospitals in the Tokai region with a diagnosis of AMI between 2001 and 2003. A total of 1837 patients (62 with dementia and 1775 without dementia) with AMI, aged 65 and over, were included in the present analysis. Patients with dementia were in general older, female, and impaired in their daily activities. They were also more likely to have a history of myocardial infarction, heart failure, cerebrovascular disease, and less likely to have a history of angina or smoking. They were less likely to have chest pain on arrival and lateral myocardial infarction. The percentage of patients with dementia who were transferred to an intensive care unit/coronary care unit or who were given percutaneous coronary intervention was lower. At discharge, the percentage of patients with dementia treated with aspirin was lower, and that of patients with dementia treated with diuretics was higher. In-hospital death rates for patients with and without dementia were 17.7% and 11.1% during hospitalization, respectively (P = 0.101). Long-term mortality after AMI was higher among patients with dementia before adjustment (24.2% versus 14.6%, P = 0.004). However, we were unable to detect differences after adjustment for potential confounders. Thus, our findings suggest that dementia has minimal effects on long-term mortality in patients with AMI.