Interatrial block (IAB) is associated with atrial fibrillation (AF) in different clinical situations, but little information exists in elderly patients with myocardial infarction (MI) and its association with frailty. Consecutive MI patients aged ≥75years were prospectively included. Frailty was assessed during the admission, as well as the prevalence of IAB. Main outcome measure was mortality and new onset AF at one year. We included 254 patients. From 220 patients with sinus rythm (86.6%), 37 had partial IAB (16.8%) and 34 advanced IAB (15.5%). Patients with advanced IAB had lower values of handgrip strenght (19.8 vs 21.7kg, p 0.073). These patients had a trend toward higher incidence of AF or mortality during follow up (HR 1.51, 95% CI 0.85–2.70, p=0.164). Advanced IAB was associated with a trend toward higher prevalence of frailty. Elderly patients with MI and advanced IAB had a trend toward higher incidence of AF. •Almost one of each three elderly patients with myocardial infarction in sinus rythm do not have a normal P wave on ECG.•Advanced interatrial block was associated with a trend toward more fraity, but not with the rest of geriatric syndromes.•Elderly patients with MI and advanced IAB had a slightly higher mortality and new onset atrial fibrillation at one year.