The effect of atrial contraction on left ventricular function in six patients with varying degrees of mitral stenosis was determined by utilizing the pressure gradient technique to measure instantaneous aortic blood flow and pressure. Aortic flow was measured as ventricular rate was controlled by right ventricular pacing to create A-V (atrioventricular) dissociation at varying rates (90-150 beats/min). At each heart rate, beats with preceding P waves, effective atrial systole, were grouped according to the duration of the P-R interval. Beats without P waves served as controls. There was always a significant increase in stroke volume, created by effective atrial systole, but the P-R interval at which it took place was different for each patient. There was no difference in the stroke volume for beats preceded by P waves having a P-R interval within the range of 0.05-0.20 sec. These beats were grouped for each patient, subjected to regression analysis, and compared to control beats. The absolute and percent change created by effective atrial systole was inversely proportional to the severity of the disease as determined by mitral valve orifice size. Effective atrial systole plays less of a role in augmenting left ventricular function in patients with mitral stenosis than in patients with normal valves.