SummaryA J-shaped or U-shaped curve phenomenon might exist between systolic blood pressure (SBP) or pulse pressure (PP) at admission and in-hospital mortality in Japanese patients with acute myocardial infarction (AMI) undergoing primary percutaneous coronary intervention (PCI). However, data regarding a relationship between mean blood pressure (MBP) at admission and in-hospital outcome in AMI patients undergoing primary PCI are still lacking in Japan.A total of 1,413 primary PCI-treated AMI patients were classified into quintiles based on admission MBP (< 79 n = 283, 79-91 n = 285, 92-103 n = 285, 104-115 n = 279, and ≥ 116 mmHg n = 281). Patients with MBP < 79 mmHg had a significantly higher in-hospital mortality, while mortality was not significantly different among the other quintiles: 16.6% (< 79), 4.9% (79-91), 3.9% (92-103), 3.2% (104-115), and 5.0% (≥ 116 mmHg). On multivariate analysis, Killip class ≥ 3 at admission, LMT or multivessels as culprit lesions, admission MBP < 79 mmHg, and age were independent positive predictors of in-hospital mortality, whereas hypercholesterolemia and TIMI 3 flow before/after PCI were negative predictors, while the other MBP categories were not.These results suggest that admission MBP < 79 mmHg might be associated with in-hospital death, and the in-hospital prognostic effects of MBP, the steady component of blood pressure, at admission might be different from those of SBP or PP, the pulsatile component of blood pressure, at admission in Japanese AMI patients undergoing primary PCI. (Int Heart J 2016; 57: 547-552) Key words: Systolic blood pressure, Pulse pressure, Prognosis, J-curve phenomenon B ased on numerous reports from Western countries, an inverse relationship between systolic blood pressure (SBP) at admission and in-hospital mortality in acute myocardial infarction (AMI) patients has largely been established, [1][2][3][4] and indeed admission SBP is involved in several riskscoring methods for AMI patients.5-7) In addition to major blood pressure indexes, such as SBP and diastolic blood pressure (DBP), blood pressure is characterized by its pulsatile and steady components, which correspond to pulse pressure (PP) and mean blood pressure (MBP), respectively. We have already shown that admission SBP or PP in a certain interval between an extremely low subset and extremely high subset might be associated with a lower in-hospital mortality in Japanese AMI patients undergoing primary percutaneous coronary intervention (PCI), which followed a J-or U-shaped curve pattern. 8,9) However, sufficient data regarding the effects of the steady component of blood pressure, MBP, at admission on the prognosis in AMI patients are still lacking.