Data from the London Marathon, with 650,000 completed runs, show that cardiac arrests occur even in the most experienced runners. Although coronary artery disease was the commonest cause of sudden cardiac arrest (SCA) with five deaths and six resuscitations, hypertrophic cardiomyopathy or idiopathic left ventricular hypertrophy (HCM) was diagnosed at autopsy on three occasions. HCM deaths had the same average age as the runners with ischaemic heart disease who had SCA or sudden cardiac death. The cardiac arrests were at the finish in less than one-third of cases and the remainder occurred between 6 and 26 miles on the course. Only one of the eight runners who died had reported symptoms to his family or physician suggestive of cardiac disease. The runner who had reported pre-race angina pain was investigated with a negative exercise stress test prior to the marathon and despite this died with a left anterior descending coronary artery stenosis. The cardiac death rate for the London Marathon is 1 in 80,000 finishers.