Background: Primary percutaneous coronary intervention (PCI) is an important treatment option for patients with acute myocardial infarction (MI). Although an inverse association between a hospital's PCI volume and in-hospital mortality has been observed in Western studies, previous Japanese investigations have not found any such relationship. Methods and Results: A retrospective analysis of 8,391 cases of acute MI, obtained from administrative data from 2006. The primary outcome was in-hospital mortality. Hospitals were divided into quartiles based on the number of PCI procedures per half-year (6-13, 14-22, 23-38, 39-134) and mortality rates were compared across the groups. Crude-mortality in the lowest-volume quartile was 7.0%, compared with 4.9% in the highest-volume quartile. An inverse association was found between primary PCI procedure volume and crude in-hospital mortality (P=0.016). After case-mix adjustment, a significant decrease in mortality risk for patients treated at high-volume (3^rd and 4^th quartile) hospitals compared to the lowest-volume (1^st quartile) hospitals was found. Conclusions: Based on this administrative data, there is an inverse association between a hospital's primary PCI volume and in-hospital mortality for patients with acute MI. Periodic outcomes research is necessary in conjunction with progress in PCI practice and technology to establish the recommended PCI volume and regionalization for improvements in care. (Circ J 2011; 75: 1107-1112)