The physiochemical characteristics of hydroxyethyl starch make it suitable for use as a colloidal blood plasma substitute. In high doses, this drug may interfere with blood coagulation. Because of its effectiveness and low cost, we have used hydroxyethyl starch rather than albumin in the priming fluid for cardiopulmonary bypass: 500 ml of 6% hydroxyethyl starch and 2,000 ml of lactated Ringer's solution. To determine if excessive bleeding has been associated with the use of hydroxyethyl starch, we reviewed 760 cardiac operations. The patients were 49.9 +/- 0.5 years old (mean +/- SEM) and weighed 73 +/- 1 kg. Blood loss during the first postoperative day was 578 +/- 25 ml, and 4.0 +/- 0.2 units of bank blood were utilized in the perioperative period. We have used an improved method of administering heparin and protamine for the past 3 years. In the 461 patients operated upon since then, blood loss was 437 +/- 21 ml, 2.9 +/- 0.1 units of bank blood were used, and excessive postoperative bleeding necessitated re-exploration in nine patients (2.0%). These results compare favorably with other recently published series in which hydroxyethyl starch was not used in the pump prime. Thus the dose of hydroxyethyl starch in our priming fluid does not appear to be associated with excessive bleeding. In view of its safety and low cost, hydroxyethyl starch is a suitable colloidal blood plasma substitute for use during cardiopulmonary bypass.