We studied patients undergoing A-C Bypass with haemodilution. The bubble oxygenator was primed with either two units of plasma and lactated Ringer's solution (PLASMA) or with lactated Ringer's solution (RINGER'S) alone. We found no difference in pulmonary function or lung water determinations between the Ringer's and the Plasma groups. When all patients were grouped together we found no significant increase in lung water (ETVL)5 immediately post-operatively. This suggests lung water did not increase during cardiopulmonary bypass. By the following morning there was a highly significant increase in lung water which was related to changes in the balance between hydrostatic pressure and colloid osmotic pressure and which continued through the second post-operative day. Pulmonary dysfunction, as measured by gas exchange, was not a major problem except in one patient whose lung water increase exceeded 3 ml H2O/kg of total body weight (52 per cent increase) and in whom there was increased shunting. We consider the increase in lung water important, since it may reduce lung compliance and further lung water accumulation. Since the increased lung water was associated with a change in the balance of Starling's forces (PMV - COPMV), treatment designed to reduce that balance should reduce lung water accumulation.