© 2016 Japan Geriatrics SocietyAim: Octogenarian patients have higher mortality and morbidity rates after acute coronary syndromes. Risk factors for in-hospital mortality in the primary percutaneous coronary intervention (PCI) era were underrepresented in previous studies. In the present study, we aimed to assess the risk factors of in-hospital mortality after primary PCI in this population. Methods: We analyzed 2353 patients who underwent primary PCI after ST segment elevated myocardial infarction (STEMI). Patients were divided into two groups according to aged: ≥80 years (octogenarian) and <80 years (control). Risk factors for in-hospital mortality were analyzed in the whole group and octogenarian patients. Results: We found that octogenarians have 10.6-fold higher mortality risk after STEMI. After a univariate and multivariate analysis, acute stent thrombosis was the most explicit risk factor for in-hospital mortality in the octogenarian group (OR 21.13, 95% CI 2.11–102.76, P < 0.001). Additionally, anterior myocardial infarction (OR 4.90, 95% CI 1.90–22.10, P = 0.04), ventricular arrhythmias (OR 15.64, 95% CI 2.81–87.12, P = 0.002), multivessel disease (OR 6.5, 95% CI 1.11–38.85, P = 0.04), ejection fraction <30% (OR 1.24, 95% CI 0.26–6.00, P = 0.04) and KILLIP score ≥2 (OR 1.20, 95% CI 0.20– 7.41, P = 0.01) were also significantly associated with mortality. Conclusions: Acute stent thrombosis, anterior MI, heart failure, low ejection fraction, ventricular arrhythmias and multivessel disease are the independent risk factors for in-hospital mortality among octogenarian patients after primary PCI. Geriatr Gerontol Int 2017; 17: 584–590.