Spatial characteristics of the QRS and T loops in 110 patients with old myocardial infarction were analysed in comparison with 221 normal subjects. Measured were (1) QRS and T polar vectors, (2) initial 20- and 30-msec segmental QRS polar vectors, and (3) length, width, thickness, and ratios of width/length and thickness/length of the QRS and T loops in edgewise and broadside projections. Broadside and edgewise projections were obtained by transformation of the reference frame of the Frank lead system to a patient's own frame based on the polar vector. The recognition rates of abnormality in the QRS and T polar vectors were 66% and 60% of a total of 110 patients with myocardial infarction. The initial 30 msec segmental QRS polar vector showed the highest recognition rate of abnormality in myocardial infarction, i.e., 87% in anterior myocardial infarction, 100% in extensive anterior myocardial infarction and 78% in inferior myocardial infarction. The initial segmental QRS polar vector was abnormally deviated posteriorly and superiorly in inferior myocardial infarction. In anterior myocardial infarction, the initial segmental polar vector was directed inferiorly in more than 50% of the cases, while the vector in normal subjects was located superiorly and to the left. The QRS loop of anterior myocardial infarction was significantly smaller in the width and width/length ratio and significantly larger in the thickness and thickness/length ratio than those of the normal. Poor planarity of the QRS loop was one of the characteristics of myocardial infarction, especially of extensive anterior myocardial infarction. The T loop of myocardial infarction was significantly larger in the width/length ratio than that of the normal. More than 50% of the cases with anterior myocardial infarction showed abnormally wide T loops. The polar vector was a useful index to characterize the spatial orientation and sense of rotation of the spatial loop. In addition, the initial segmental QRS polar vector represented the mild localized abnormalities of the spatial loop. The loop configuration in space was characterized in edgewise and broadside projections.