Automatic techniques for interpreting the electrical activity of the heart are based more and more on vectorcardiographic parameters, especially information provided by the spatial vectorcardiographic loop. This data has been shown to be a useful complement to classical electrocardiography. The aim of this study was to define a method of calculation of the planes of the vectorcardiographic loops of depolarisation and repolarisation, and to calculate a coefficient of left-sidedness obtained by the sum of squares of the distances between the points on the loop in the plane. This value is then normalised with respect to the size of the loop. Normal values of this coefficient were first defined in a healthy reference population of 70 subjects: the values are expressed in (MV/10)2 or in mm2, and are 0,28 +/- 0,05 for the QRS and 0,0026 +/- 0,0008 for the ST-T. The coefficient was then calculated in different pathological groups, the diagnosis of which had been formally confirmed: ventricular hypertrophy, valvular heart disease, conduction defects, coronary artery disease. The highest values (four times normal) were obtained in right ventricular hypertrophy, right bundle branch block and infarction associated with conduction defects. The discriminative value of the coefficient of left-sidedness is discussed with the aim of distinguishing the normal from the pathological.