Objective: To assess the utility of left bundle branch block (LBBB) as an activation criterion for primary percutaneous coronary intervention (PPCI). Design Retrospective observational cohort study. Setting Single UK heart attack centre. Patients Consecutive patients referred for PPCI September 2008-December 2011 (n=2192). Interventions Demographic and outcome data were obtained by review of case notes, angiograms and interrogation of local/national databases. Main outcome measures Angiographic culprit lesion assessment defined appropriate and inappropriate activations. Patients outcomes were assessed by Major adverse cardiac events (MACE), defined as a composite of mortality and unplanned revascularisation at 1-year. Results: LBBB-activation occurred in 120 patients (5.5%), of whom 21 (17.5%) had acute coronary occlusion angiographically, and were adjudicated appropriately. Compared with appropriate activations for ST segment elevation, appropriate LBBB-activations were older (71.0±9.6 vs 64.2±12.4 years, p=0.01) and more likely to be in cardiogenic shock (19.0% vs 4.3%, p<0.01). Extent of disease quantified by the SYNTAX score did not differ (median 21.5, IQR 11.0-27.0 vs 19, 11.0-25.5, p=0.66), but amount of myocardium at-risk was higher in appropriate LBBB-activations (culprit jeopardy score median 4, IQR 2-6 vs 2, 2-4, p=0.02). Final diagnoses for LBBB-activations were acute coronary syndrome (39.2%), non-acute coronary syndrome cardiac chest pain (33.3%) and non-cardiac chest pain (27.5%). In appropriate LBBB-activations 1-year mortality and MACE were higher (23.8% vs 6.6%, p=0.002 and 28.6% vs 10.5%, p=0.007, respectively). Conclusions: Our data suggests that despite its poor specificity for identifying acute coronary occlusion, LBBB should at the present time remain an activation criterion for PPCI and such patients should continue to be transferred to heart attack centres for assessment and treatment.