This study further extends the clinical usefulness of the estimation of cardiac chamber size by means of single echocardiographic dimensions. In 24 patients left atrial size was calculatedfrom an anteroposterior standard echographic dimension and correlates (r=o088) with the left atrial surface area measured from selective cineangiograms in the right anterior oblique position. For clinical use a left atrial aortic dimensional ratio was derived as a valid index for the separation of normal and enlarged atrial cavities. An excellent correlation between angiographic and echographic derived volumes wasfoundfor left ventricular volumesfrom single left ventricular echographic dimensions in the 35 patients studied. Correlation coefficients for end-systolic and end-diastolic volumes were r=o g96 and r=0o97, respectively. As might be expected, calculated stroke volumes and ejection fractions from echocardiograms and angiograms correlated less well (r==0 82 and r= o 79, respectively). From these results andfrom studies published by others, despite theoretical limitations and several assumptions in the use of single left ventricular dimensions, it is concluded that calculations can be used with confidence for volume determinations in the majority ofpatients, including those with coronary artery disease and over a wide range of left ventricular dimensions and sizes. It appears that, provided these observations are extended, derived variables such as stroke volume and ejection fractions, can be obtainedfor the assessment of left ventricular function by non-invasive means.