BACKGROUND In the current era of primary percutaneous coronary intervention (PCI), some patients with acute myocardial infarction (AMI) still do not undergo primary PCI. METHODSANDRESULTS To examine the clinical characteristics of AMI patients who did not undergo primary PCI, we analyzed patients enrolled between 2002 and 2010 in the MIYAGI-AMI Registry Study, in which all AMI patients in the Miyagi prefecture have been prospectively registered. Among a total of 8,640 patients, 1,879 (21.7%) did not undergo primary PCI and their in-hospital mortality was significantly worse compared with those who did (21.4% vs. 6.4%, P<0.01). Multivariate analysis demonstrated that female sex was significantly associated with non-performance of primary PCI [odds ratio (95% confidence interval): 1.40 (1.22-1.61), P<0.001], along with age [1.01 (1.01-1.02), P<0.001] and heart failure on admission [2.69 (2.29-3.16), P<0.001]. When dividing by age, the non-performance rate of primary PCI in females showed a U-shaped prevalence, whereas it simply increased with aging in males. Importantly, female patients aged <80 years had a significantly higher non-performance rate of primary PCI compared with male patients, regardless of the severity of AMI. CONCLUSIONS These results indicate that in the current PCI era, various factors, including aging, heart failure on admission and sex differences, are associated with non-performance of primary PCI, which remain to be resolved in order to further improve critical care of AMI.