In a series of 51 clinico-pathological examinations on patients who died during the first 15 days after the onset of clinical symptoms of their first and only transmural myocardial infarction (anterior: 29 cases; posterior: 22 cases) the causes of death were divided into: heart failure -- 26 cases (53 p. 100); rupture of the heart -- 22 cases (43 p. 100); disorders of ventricular rhythm -- 2 cases (4 p. 100). The anatomical basis of fatal cardiac failure is twofold: either a very extensive area of necrosed muscle, of poor quality of the mass of muscle not involved in the infarction. In the anterior infarctions (16 cases, representing 55 p. 100 of deaths in this group) the first factor was foremost, the mean extent of necrosed muscle constituting 42 p. 100 of the total left and septal ventricular mass; stenotic coronary lesions, which were commonly found on the anterior descending artery, were confined to this artery alone in 10 cases 62 (p. 100). In posterior infarctions (11 cases, representing 50 p. 100 of deaths in this group), the mass of muscle destroyed was less (mean 36 p. 100), but the stenotic coronary lesions were diffuse, involving the three main trunks in 9 cases, which also explains the poor quality of the muscle not involved by necrosis. Thus there is a clear difference between anterior and posterior infarctions followed by deaths from cardiac failure: in the first group, the remaining muscular mass is quantitatively insufficient to maintain the haemodynamics, while in the second it is qualitatively insufficient, because of poor blood supply, to maintain an adequate cardiac output.