The “B” lines of Kerley (Fig. 1) are short horizontal opaque lines seen on a chest radiograph in the lung bases just above the costophrenic angle. They are rarely longer than 1 inch and usually about 1 mm. in width. They occur, as a rule, in groups, the distance between one line and another being not more than a few millimeters. They are found in many conditions associated with obstruction to the lymphatic and venous drainage of the lungs. Gough (10) has shown by paper-mounted sections that these lines represent edema of the connective tissue of the interlobular septa and marked distention of the lymphatics passing through them. The lines may be transient, seemingly appearing when the hydrostatic pressure in the capillaries and veins increases above the level of the osmotic pressure of the blood. This supposition, however, is still only theoretical and has not been proved in animals or in man. These lines were first described by Kerley (14), who observed them in patients with anthracosilicosis. They have been shown to occur also in such conditions as mitral heart disease, severe aortic stenosis, coarctation of the aorta, sarcoidosis, lymphatic spread of metastatic carcinoma to the lungs, and in the vicinity of lung cancer. They have not been reported in patients with pulmonary arterial hypertension, which fact is of some interest in the differential diagnosis of many chest diseases. Inasmuch as the “B” lines are most commonly seen in patients with mitral valve disease, we have undertaken to correlate the mean left atrial pressure with their appearance and also with left atrial size. Methods and Materials One hundred and ten patients with rheumatic heart disease were studied by left atrial puncture. In S9 of these, satisfactory pressure curves were obtained. This group includes persons who were being considered for open or closed heart surgery and who had a variety of lesions of both the mitral and aortic valves. Prior to left atrial puncture, the patients were hospitalized for variable periods of time in an effort to reach a state of maximum cardiac compensation. The data to be reported here are part of a larger study in which the cardiac output, left ventricular pressures, valve resistance, and cross-sectional areas were also calculated. Left heart catheterization was carried out by the Fisher (7) modification of the Björk (1) technic. With the patient prone, the left atrium was visualized fluoroscopically, an 8-inch, thin-walled, 17- or 18-gauge stylated needle was inserted into it percutaneously, and polyethylene tubing was passed into the atrium and attached to a Statham gauge. The pressure measurements were made on an Electronics for Medicine photographic recorder. The normal left atrial pressure for this method varies from 4 to 8 mm. Hg.