Although the measurement of left atrial diameter (LAD) is a standard part of anyechocardiographic examination, the normal range for adults has never been well established or correlated with body surface area (BSA) and sex. We studied 100 males and 100 females whose ages ranged from 15 to 70 years, with no evidence of mitral value disease or other form of heart disease which might cause left atrial enlargement. All measurements were obtained between the external surface of posterior aortic root (AR) and the internal surface of the left atrial wall and were recorded at ventricular end diastole (ED) as well as end systole (ES). The LAD at ED ranged from 9.5 to 29.5 mm with a mean of 19 mm ± 5.0 S.D.; the diameter at ES ranged from 18.3 mm to 38.7 mm with a mean of 28.5 mm ± 5.1 S.D. The mean LAD at ED was 20.7 mm ± 4.8 S.D. in males compared to the mean diameter of 18.3 ± 4.9 S.D. in females which represents a significant difference (p<0.001). The LAD did not correlate with BSA. The left atrial dimension by ultrasound in these 200 normal patients was compared with the same measurement in 50 catheterized patients with mitral valve disease and proven left atrial enlargement. When absolute values of the left atrial dimension both at end systole and end diastole were determined by ultrasound, there was a clear separation between normal and abnormal (p<0.0001).