Air embolism--the most dangerous complication of central venous catheterization--may occur in several ways. The most frequent is from disconnection of the catheter from the related intravenous tubing. An embolism may present with a sucking sound, tachypnea, air hunger, wheezing, hypotension and a "mill wheel" murmur. A later manifestation is severe pulmonary edema. In a review of 24 patients, the mortality was 50 per cent. Among the survivors, five (42 per cent) had neurologic damage. Immediate treatment includes placing the patient in the left lateral and Trendelenberg positions, administration of oxygen and aspiration of air from the heart. Cardiac massage and emergency cardiopulmonary bypass may be necessary. Most instances can be prevented by inserting the cannula with the patient in the Trendelenberg position, occluding the cannula hub except briefly while the catheter is inserted, fixation of the catheter hub to its connections and occlusive dressing over the track after removal of the catheter.