Objective To investigate the effects of hydroxyethyl starch 130/0.4 (HES) used as priming fluid for cardiopulmonary bypass (CPB) on the plasma colloid osmotic pressure (COP) and lactic acid (LAC) concentration in infants undergoing cardiac surgery.Methods Forty infants of either sex with age ≤6 yr undergoing cardiac surgery with CPB were randomly divided into 2 groups (n =20 each): HES group and control group. The left radial artery and the right internal jugular vein were cannulated for blood pressure (BP) and the central venous pressure (CVP) monitoring. Arterial blood gases, blood LAC concentration, hemoglobin (Hb), hematocrit (Hct), mean arterial pressure (MAP) and nasopharyngeal temperature were measured and recorded immediately before and 5 min after aortic cross-clamping, at the end of CPB and operation. Plasma COP was measured before induction of anesthesia (T1), at 5 and 30 min of CPB (T2 and T3, respectively), before routine ultra-filtration (T4), at the end of CPB (T5) and2 h in ICU (T6).Results The plasma LAC concentration was significantly lower and the COP significantly higher in HES group than in control group (P＜0.05 or 0.01).The plasma LAC concentration increased after aortic cross-clamping, reached the peak at the end of CPB and then declined at the end of operation, but was still higher than that before aortic cross-clamping in both groups. Plasma COP was significantly decreased during CPB as compared with the baseline at T1, but increased at T6 in both groups.Conclusion Using HES 130/0.4 as pdming fluid for CPB can effectively improve plasma COP and reduce blood LAC level in infants undergoing cardiac surgery with CPB.