The relationship between R-wave amplitude and left ventricular volume was examined using two groups of patients, undergoing diagnostic cardiac catheterisation for investigation of chest pain, who had simultaneous R-wave recording and left ventricular angiography. R-wave amplitude was measured in leads 1, 2, 3 and V4–6. Left ventricular volume was altered by nitroglycerine (n = 18) and atrial pacing (n = 13). In both groups, increase or decrease in left ventricular volume was associated with a concomitant change of R-wave amplitude. We conclude that left ventricular volume is an important determinant of surface-recorded R waves with increased amplitude reflecting increased left ventricular volume and vice versa.