Colloid solution is commonly used to increase the oncotic pressures of priming solutions used in the cardiopulmonary bypass circuit. To study the effectiveness of this practice, we prospectively randomized 100 adult patients undergoing cardiac operations to receive Ringer's lactate solution plus 50 gm of albumin (group A) or Ringer's lactate solution alone (group B) as the prime solution for the bypass circuit. Personnel involved in the management of these patients were blinded concerning the group to which the patients had been randomized. Forty clinical parameters related to perioperative fluid balance, cardiopulmonary function, and renal function were studied. Although group B received a larger volume of crystalloid solution intraoperatively (p less than 0.05), had a lower mean cardiac filling pressure (p less than 0.05), and had a higher hematocrit value (p less than 0.05) in the immediate postoperative period, all mean values for both groups were within the normal range. There were no differences between the two groups with regard to postoperative clinical parameters of cardiopulmonary and renal function, nor was outcome affected by the addition of albumin to the prime solution. We conclude that there is no clinically detectable advantage for the practice of adding 50 gm of albumin to the priming solution of bypass circuits in adults undergoing cardiac operations. Routinely supplementing the bypass prime solution with albumin adds significant cost, estimated to be approximately $10,000 per 100 cases, without demonstrable clinical benefits. Whether this practice can be of value in selected cases needs to be further studied.