A youth of 16 years of age died suddenly and quite unexpectedly while walking to school. The necropsy disclosed a quadricuspid aortic valve with complete isolation of the left coronary artery by an adherent aortic valve cusp. The left ventricular myocardium showed subendocardial contraction band necrosis suggesting that critical ischaemia had triggered a state of hypercontraction, in keeping with ventricular fibrillation as the immediate cause of death. The site of the anomaly in the aortic root showed dysplasia of the aortic wall and the affected valve cusp, histologically similar to the changes that characterise supravalvar aortic stenosis. The findings suggest a developmental anomaly, possibly a forme fruste of supravalvar aortic stenosis, rather than a postnatally acquired condition.