Sarcomere measurements in the infarcted zone at sudden death, outside the infarcted zone and without acute infarction in the myocardium of patients with ischaemic heart disease (IHD) and in patients decreased due to other causes showed that sarcomere relaxation occurs in extensive areas of muscle fibres already in the earliest stage of infarction, and is not found outside the area of acute ischaemia. This phenomenon can be revealed even without sarcomere measurement, by analysing histological material stained by routine methods in normal and polarized light. This facilitates the diagnosis of acute myocardial infarction in subjects who have died suddenly.