Pulmonary artery-bronchus ratios in patients with normal lungs, pulmonary vascular plethora, and congestive heart failure. The pulmonary artery-bronchus ratio (ABR), defined as the external diameter of a pulmonary artery divided by the external diameter of its accompanying bronchus, was measured in the upper and lower lung zones on frontal chest radiographs in the following groups of subjects: erect healthy subjects, erect subjects with pulmonary vascular plethora, erect subjects with clinical evidence of decompensated left-sided congestive heart failure (CHF), supine normal subjects, and supine subjects with clinical evidence of decompensated CHF. Characteristic vascular patterns were found in each group. The positive predictive value of an abnormal vascular pattern determined by means of changes in the ABR on the erect chest radiograph was 100%, with the negative predictive value of a normal pattern being 97%. The erect chest radiograph was 93% accurate in distinguishing between plethora and CHF. The positive predictive value of an abnormal vascular pattern on the supine chest radiograph was 100%, with the negative predictive value of a normal vascular pattern being 97%. Measurements of ABR can be a helpful adjunct to the interpretation of supine and erect chest radiographs. 