Twenty-six patients were subjected to large intraoperative shifts in plasma colloid oncotic pressure (COP) by dilutional cardiopulmonary bypass, salt poor albumin (SPA), and/or diuretic therapy. During bypass, COP fell 44%, to a level that leads to pulmonary edema in vitro. Intraoperative changes in COP, induced by the postbypass use of SPA and diuretics, were associated with increased COP and urine output, but did not correlate with changes in alveolar-arterial oxygen tension gradients (A-a Po2). We conclude that therapy designed to change COP has a limited role and does not affect intraoperative pulmonary function as measured by A-a Po2 gradients.