In 30 consecutive children undergoing cardiac surgery, two different types of fluid were given randomly for volume replacement in the pre-bypass period. In group 1 (n = 15), low molecular weight hydroxyethyl starch solution (LMW-HES) (6% HES; mean molecular weight 200,000 Da, molar substitution 0.5) and in group 2 (n = 15) 20% albumin (HA) was infused from the induction of anaesthesia until the start of cardiopulmonary bypass (CPB). In addition to haemodynamic values, various laboratory variables were measured before and after CPB until the morning of the 1st day after operation. The patients did not differ in diagnosis and conduct of CPB (lowest rectal temperatures: group 1 29.0 (SD 1.1) degrees C; group 2 29.4 (1.0) degrees C). Haemodynamic data (MAP, HR, CVP), anti-thrombin-III, fibrinogen, platelet count and coagulation variables were comparable between the groups until the 1st day after operation. Postoperative blood loss and the use of homologous blood or blood products were similar in all children. Albumin concentration increased after infusion of albumin (35-47 g litre-1) and was significantly greater until the end of the operation compared with the LMW-HES-treated children. Colloid osmotic pressure, however, was similar in the two groups and returned to baseline values on the 1st day after operation (LMW-HES group 19.31 (1.2) mm Hg; HA group 18.0 (1.3) mm Hg). Post-bypass urine output and creatinine values also did not differ between the groups. Anaphylactic reactions were not observed in any of the patients. It can be concluded that LMW-HES solution can be used effectively and safely for volume replacement in the pre-bypass period in small children undergoing cardiac surgery.