The electrocardiograms before and after operation have been compared in 20 patients undergoing left mastectomy and in 19 patients undergoing right mastectomy.There are highly significant increases in the amplitude of the QRS complexes recorded over the precordium after left mastectomy, except in lead V6.There are highly significant increases in the amplitude of the QRS complexes of leads V3Rand V1after both left and right mastectomy.These data are not completely explained by a diminution of distance between the recording electrode and the myocardium or of electrical resistance following the excision of soft tissue.The possible relationship is suggested between the precordial voltage changes and the altered thoracic volume conductor.