The treatment for pneumothorax varies from invasive chest tube drainage to conservative management with bed rest. Based on the behavior of gases in closed body cavities, the inhalation of supplemental oxygen hastens absorption of the pneumothorax. To evaluate oxygen as a therapeutic agent, we treated 8 patients with pneumothoraces of various degrees with a high concentration of inspired oxygen delivered by a partial rebreathing mask. Periodic roentgenograms were used to measure the change in size of pneumothorax in order to assess the rate of resolution. 6 patients with pneumothoraces of less than 30% showed a mean resolution rate of 4.2% per day with reduction to one-third original size in the first 72 h. This was more than three times the rate of resolution (1.25% per day) previously reported with breathing room air alone. In 2 patients who initially received a lower concentration of inspired oxygen via nasal cannula, the rate of absorption increased after placing them on a partial rebreathing mask. 2 patients with four episodes of pneumothoraces greater than 30% did not benefit from this form of therapy and eventually needed chest tube drainage. We conclude that the administration of high concentrations of inspired oxygen is an effective method to enhance the rate of resolution of pneumothoraces, particularly when smaller than 30%, thereby reducing morbidity and duration of hospitalization and avoiding invasive drainage procedures.