Objectives: To determine the length of catheter required to perform a needle thoracostomy, as determined by chest wall thickness, to treat the majority of patients presenting to the emergency department (ED) with a potential tension pneumothorax. Methods: A convenience sample of 111 computed tomography (CT) scans of the chest in trauma and medical resuscitation patients at a military Level 1 trauma center in San Antonio, Texas, was pooled, and the chest wall thickness was measured at the second intercostal space, midclavicular line, to the nearest 0.1 cm. Results: The mean chest wall thickness in the patients studied averaged 4.24 cm (95% confidence interval [CI] = 3.97 to 4.52). Nearly one fourth (25) of the study patients had a chest wall thicker than 5 cm. Women, on average, have thicker chest walls than men (4.90 for women; 4.16 for men; p = 0.022).Conclusions: In this study, a catheter length of 5 cm would reliably penetrate the pleural space of only 75% of patients. A longer catheter should be considered, especially in women.