Tension pneumothorax as a complication of ventilatory support may cause severe cardiac problems. The diagnosis may be difficult. Risk factors predisposing to the misdiagnosis of this condition include atypical location of the pneumothorax. Three patients with adult respiratory distress syndrome who had atypical, localized tension pneumothorax are described. Each had an ipsilateral functioning chest tube in place at the time. Placement of chest tubes into the localized pneumothoraces resulted in immediate improvement in hemodynamic status in two patients, but the third patient died before the chest tube could be placed. A diagnosis of tension pneumothorax should be considered in any patient on a ventilator whose hemodynamic status deteriorates in the presence of high airway pressures. In patients with adult respiratory distress syndrome the pneumothorax can remain localized because the heavy, noncompliant lungs cannot collapse enough for air to dissect diffusely through the pleural space; pleural adhesions may also contribute to this phenomenon.