To evaluate the value of decubitus chest radiography in the clinical assessment of pneumothorax, the authors compared the detectability of pneumothorax on expiratory, upright chest radiographs with that on expiratory, lateral decubitus radiographs obtained with the suspected side up. Five radiologists reviewed 19 sets of radiographs with pneumothoraces and 20 sets of radiographs without pneumothoraces. All five radiologists were more accurate and had greater confidence in the evaluation of the upright studies. A pneumothorax was detected 21% (77 versus 57 of 95) more often on the erect radiographs. In three cases, however, decubitus positioning was more definitive. These results suggest that when clinically feasible, the expiratory, upright chest radiograph is the procedure of choice for the evaluation of small pneumothoraces.