4,071 consecutive cardiac catheterizations were reviewed retrospectively to obtain 56 cases of pure mitral regurgitation among whom chest X-ray, electrocardiography and echocardiography had been performed within 10 days of catheterization. Mitral regurgitation was mild to moderate (1-2+) in 7 of the 56 cases while 49 had more severe regurgitation (3-4+). Left atrial size as measured by echocardiography was found to be enlarged in all but 2 cases. A positive correlation between left atrial size and severity of mitral regurgitation, irrespective of the height of the pulmonary capillary wedge pressure V wave was demonstrated. There was no demonstrable, predictive relationship between left atrial size and pulmonary capillary wedge pressure or electrocardiographic or chest X-ray findings. We conclude that knowledge of left atrial size is predictive of the severity of mitral regurgitation; however, it is not possible to predict the pulmonary capillary wedge pressure or height of the V waves based on a knowledge of left atrial dilatation in patients with pure mitral regurgitation.