PURPOSE--To evaluate changes in the electrocardiogram (ECG) with incorrect positioning of the chest lead for V1, placed in the second and third right intercostal spaces. METHODS--Two hundred and five patients were studied after a conventional ECG, with the record of tracings where the chest electrode for V1 was placed at the second and third right intercostal spaces at the right sternal border. These tracings were then compared with the former one and changes observed in the P wave, QRS complex and T wave registered and submitted to statistical analysis. Patients age ranged from 6 to 89 years, mean 46. Whites comprised 79 per cent , black 6 per cent and mulattos 15 per cent . Women totalized 62 per cent and men 38 per cent of the sample. Clinical diagnosis were arterial hypertension (50 per cent ), no apparent cardiac disease (41 per cent ), coronary atherosclerotic heart disease (4 per cent ), mitral valve prolapse and other organic heart disease (5 per cent ). RESULTS--Negativation or accentuation of negative terminal forces of P wave were present in 84 per cent of the patients, alterations in the QRS complex in 75 per cent of the cases, and in the T wave in 66 per cent of the cases, with confidence intervals of 0.7898 to 0.8901; 0.6907 to 0.8092; and 0.6163 to 0.7437, respectively. CONCLUSION--Important changes in the ECG record may occur with high probability if the chest electrode for lead V1 is placed above the standard place