Examination of more than 30 cases of bronchopleural fistula (BPF), of diverse causes, including 6 following resectional surgery, revealed a distinctive configuration of air/fluid collections in the pleural space. Maler in 1940 independently observed that loculated BPF pockets conform in shape to the adjacent chest wall. With the most common posterior costophrenic angle location, there is a wide air-fluid level in the frontal view, but on lateral films the anteroposterior diameter is narrow. In contrast, abscess cavities tend to be spherical and farther from the ribs. Use of these plain film criteria permits earlier and more confident diagnosis.