Findings in 30 consecutive patients with pneumothorax were prospectively analyzed. All patients underwent supine anteroposterior (AP) and erect posteroanterior (PA) chest radiography. The interpleural distances were measured at three locations. Average interpleural distances on supine AP and erect PA radiographs were analyzed by means of correlation and linear regression analysis. The authors developed a formula that can be used to estimate the size of a pneumothorax on a supine AP chest radiograph.