The effects on blood glucose concentrations of packed red blood cells (AS-1) (group I) versus washed red blood cells (group II) for cardiopulmonary bypass prime were compared in 20 infants weighing less than 10 kg undergoing cardiac surgical procedures. All patients were anesthetized with N2O/O2/isoflurane/fentanyl and received lactated Ringer's solution prior to bypass. Blood glucose concentrations prior to bypass were 85 ± 15 mg/dl (mean + SD) in group I and 81 ± 14 mg/dl in group II. Blood glucose concentrations were 210 ± 21 mg/dlversus78 ± 14 mg/dl (P < 0.001) 10 min after initiation of bypass, 241 ± 48 mg/dlversus107 ± 28 mg/dl (P < 0.001) prior to separation from bypass, and 214 ± 52 mg/dlversus97 ± 19 mg/dl (P < 0.001) after protamine administration in group I and group II, respectively. The use of washed red blood cells for cardiopulmonary bypass priming solution in infants significantly attenuates the increase in blood glucose concentration otherwise observed during cardiopulmonary bypass.