The distances between three or four radiopaque markers located on the left ventricular epicardial surface at the apex and in the region of the minor axes in 22 subjects showed close correlations with left ventricular chamber radii, length, volume and wall thickness determined from biplane angiograms over the range of stroke volume. The markers were previously placed during heart surgery. Regression equations relating the distances between epicardial markers and chamber volumes were used to predict volumes for other beats. There was close agreement of end-diastolic, end-systolic, and stroke volumes as determined by the angiographic and epicardial marker methods for premature atrial contraction beats and post premature contraction beats, and in studies performed during rest and exercise. Time-volume curves determined by the epicardial marker and angiographic methods were similar. It is concluded that epicardial marker measurements can be used to quantitate beat-to-beat left ventricular chamber volume and dimension changes.