Abstract Background The Sgarbossa score has been used to identify acute myocardial infarction on ECG in the presence of LBBB but has relied on elevated CK-MB for validation rather than angiographic evidence of vessel occlusion. Methods We determined (a) the presence or absence of Sgarbossa criteria with concordant (S-con) or discordant (S-dis) ST changes, (b) the presence of acute coronary occlusion or likely recent occlusion on angiography and (c) the biochemical evidence of myocardial infarction (Troponin T > 0.10 μg/L, Troponin I > 1.0 μg/L) in patients field-triaged with suspected AMI and LBBB. Results Between April 2004 and March 2009, 102 patients had field ECGs transmitted by paramedics for triage — 8 with S-con, 26 with S-dis and 68 with LBBB alone. Acute coronary occlusion was present in 8/8 with S-con but none of the S-dis or LBBB alone patients, and in all 8 S-con patients reperfusion resulted in resolution of S-con changes. Likely culprit lesions with TIMI 3 flow were found in 3 S-dis patients but stenting did not result in resolution of S-dis. LBBB did not resolve in any patient. Troponin was elevated in 26 patients — 11 with occlusion or likely culprit lesions, 15 with non-ischaemic causes. Conclusions In the absence of S-con, LBBB is not associated with acute coronary occlusion and should not be used as criteria for reperfusion therapy in myocardial infarction.