A very fit 28-year-old Coloured athlete presented with an acute transmural anteroseptal and non-transmural anterolateral myocardial infarction (MI). He had no significant risk factors for coronary artery disease apart from moderate cigarette smoking. Cardiac catheterization 2 months later demonstrated a significant area of myocardial damage as well as a large mural thrombus, but the coronary arteries appeared normal apart from a large irregular filling defect in the proximal left anterior descending (LAD) branch, apparently due to a thrombus. Cardiac catheterization a further 4 months later documented no further filling defect in the LAD branch and the coronary arteries appeared free of disease. Ergometrine maleate provocation on this occasion failed to demonstrate any coronary vasospasm. Possible pathophysiological mechanisms for the unexpected MI are outlined.