Twenty-one patients who underwent cardiopulmonary bypass for myocardial revascularization received postoperatively controlled mechanical ventilation (CMV) or intermittent mandatory ventilation (IMV), with or without positive end-expiratory pressure (PEEP). Functional residual capacity (FRC), right-to-left intrapulmonary shunt, dynamic compliance, physiological dead space, and arterial and mixed venous blood gases were measured during mechanical ventilation and spontaneous ventilation following weaning. Controlled ventilation increased physiological dead space and arterial pH. FRC correlated positively with dynamic compliance, but not with right-to-left intrapulmonary shunt. Postoperatively, FRC was significantly lower than normal when patients were ventilated without PEEP, but was normal when patients received PEEP. Arterial oxygen tension (PaO2), intrapulmonary shunt, and dead space were unaffected in spite of higher FRC, suggesting that patients who received PEEP had fewer atelectatic and fewer unperfused "silent" lung units than those who had not received PEEP.