65 patients after previous myocardial infarction well documented by electrocardiography were investigated by 201-Tl scintigraphy after submaximal ergometry. Topographic results of the scintigraphy in three planes were compared with the location of previous infarction demonstrated by standard electrocardiography. 30% of disagreements and partial agreements in more than half the patients were caused predominantly by inability of electrocardiography to qualify a true ischaemic necrosis in the interventricular septum, by limited ability to localize exactly the site of myocardial infarction and by underestimating its size, in particular in the posterior left ventricular wall, where the lesion often extends to the lateral wall. Some principal disagreements of both methods in different findings on the anterior and posterior left ventricular walls stress the low value of standard electrocardiography for exact topographic diagnosis of the infarct site. The authors express their opinion that the size of myocardial infarction and its influence on left ventricular function measured by non-invasive methods in the acute state would be more important for the prognosis of the patient.