Left atrium size on the frontal radiograph was assessed by measuring the distance from the middle of the "double density" to the left main bronchus in 148 normal patients and 48 consecutive patients with echocardiographic evidence of left atrial enlargement. The measurement was less than 7.0 cm in 96% of normal patients and greater than 7.0 cm in 90% of patients with left atrial enlargement. This measurement is more easily obtainable than measurements and signs requiring a cardiac series, and is usually readily available for sequential evaluations of left atrial size. It is a less reliable gauge of left atrial size in children.