Low plasma oncotic pressure (COP) following dilutional cardiopulmonary bypass (DCB) may lead to pulmonary edema. As it is difficult to normalize postbypass COP with concentrated albumin therapy, which itself causes pulmonary dysfunction, another method of hastening postbypass COP normalization was examined. Alveolar-arterial oxygen pressure gradients (A-aDO)2 of 12 patients in whom hypothermically induced, complete COP normalization was achieved within 90 minutes of DCB were compared with a previously reported normothermic, albumin-treated group in whom A-aDO2 worsened by 104 ± 22 mm Hg (P<.01). No worsening of postbypass A-aDO2 occurred in the hypothermic group. Furthermore, transthoracic impedance fell 7.9 ohms after DCB, suggesting that a rise in pulmonary water content need not widen A-aDO2 after DCB. Why hypothermia hastens postbypass oncotic normalization is uncertain. Possible mechanisms include postbypass "cold diuresis," water shifts out of the blood, or intravascular translocation of interstitial protein.