Careful histologic studies were performed on the coronary arteries, myocardium and conduction system of the hearts of six men aged 32 to 44 years who died suddenly with no history of heart disease. All six hearts demonstrated coronary atherosclerosis without evidence of complete obstruction or myocardial infarction. A nonobstructing mural coronary thrombus was found in all six hearts; in four, the thrombus was located in the left anterior descending coronary artery. Distal microthrombi were found in four hearts. In these six men, the terminal event, often a ventricular arrhythmia, may have been related to the mural coronary thrombus. Small fragments originating from such lesions can obstruct the microcirculation producing sudden lethal arrhythmias. Nonobstructing mural coronary thrombosis may be more prevalent and more significant than previously suspected and should be considered in cases of sudden cardiac death.