Hypoxemia was observed at 20 minutes and two hours postprocedure in 15 patients who had a thoracentesis. The hypoxemia resolved after 24 hours. No effect on pH, PaCO2, or bicarbonate ion was observed. The degree of hypoxemia was correlated directly with the volume of fluid removed. Oxygen therapy was found to reverse this effect. Hypoxemia that follows thoracentesis may be caused by several factors, the most important being frank pulmonary edema.