A simple measurement, the average interpleural distance (AID), which is obtained from three linear measurements, closely predicts true pneumothorax size as determined by radiographic thoracic gas volume measurement (mean variation -2.4 +/- 7.23 SD). The distribution of pneumothorax volume between the affected lung and the ipsilateral thoracic cage is variable and unpredictable. It was found that the average volume of pneumothorax that is accommodated by thoracic cage expansion (8%) is much less than previously reported. This appears to be the result of variable degrees of splinting and chest wall pain associated with pneumothorax.