Sudden ischaemic death results either from an episode of acute myocardial ischaemia consequent upon coronary thrombosis or from an arrhythmia arising within a scarred left ventricle. Very different proportions of these two groups have been reported in both clinical studies in resuscitated subjects with out-of-hospital ventricular fibrillation, and in necropsy series. In 168 cases of sudden death due to ischaemic heart disease coming to necropsy 73 (43.5%) had mural intraluminal coronary thrombi, 50 (29.8%) had occlusive intraluminal thrombi, and 45 (26.7%) had no intraluminal thrombi, giving a ratio of 2.7:1 for those with and without coronary thrombosis. Single vessel disease, the presence of acute infarction at autopsy and prodromal symptoms were positively associated with the presence of coronary thrombosis. Conversely, the presence of old myocardial infarction at necropsy, a known clinical history of ischaemic heart disease and triple vessel disease were associated with the absence of acute thrombosis. The reported variation in the incidence of coronary thrombi in sudden ischaemic death can be largely explained by selection of subjects with those clinical characteristics which are positively or negatively associated with coronary thrombosis.