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We investigated how pathologic Q waves or equivalents predict location, size and transmural extent of myocardial infarction (MI).MI characteristics, detected by contrast-enhanced magnetic resonance imaging, were compared with 12-lead electrocardiogram in 79 patients with previous first MI.Q waves involved only the anterior leads (V1–V4) in 13 patients: in all patients MI involved the anterior and anteroseptal walls and apex; 81% of scar tissue was within these regions. Q waves involved only the inferior leads (II, III, aVF) in 13 patients: in 12 of these patients MI involved the inferior and inferoseptal walls; however, only 59% of scar occupied these regions. Q waves involved only lateral leads (V5, V6, I, aVL) in 11 patients: in nine of these patients MI involved the lateral wall but only 27% of scar tissue was within this wall. Q waves involved two electrocardiogram locations in 42 patients. In the 79 patients as a whole, the number of anterior Q waves was related to anterior MI size (r=0.70); however, the number of inferior and lateral Q waves was only weakly related to MI size in corresponding territories (r=0.35 and 0.33). A tall and broad R wave in V1–V2 was a more powerful predictor of lateral MI size than Q waves. Finally, the number of Q waves accurately reflected the transmural extent of the infarction (r=0.70) only in anterior infarctions.Q waves reliably predict MI location, size and transmural extent only in patients with anterior infarction. A tall and broad R wave in V1–V2 reflects a lateral MI.