Thirty children undergoing surgical repair for congenital heart defects were randomly selected for a double-blind study on the anti-hemorrhagic and blood-saving properties of aprotinin. The treatment group comprised 14 patients who received aprotinin 7 mg/kg of body weight until the end of perfusion. The placebo group (n = 16) received an infusion of the corresponding volumes of saline. Patients treated with aprotinin bled less during the operation (12.6 ml/kg versus 18.1 ml/kg, p = 0.25) and in the first 24 postoperative hours (chest drainage 12.1 ml/kg versus 17.7 ml/kg, p = 0.07). Hemoglobin loss into chest drainage was reduced in the treated group by half (0.66 versus 1.21 gm in 24 hours, p = 0.07). Fewer blood donors were needed during hospitalization by patients receiving aprotinin (1.07 versus 2.75 donors per patient, p = 0.04). Postoperative transfusion was unnecessary in 64.2% of patients receiving aprotinin compared with only 25% of the placebo group (p = 0.03). Aprotinin increased diuresis significantly during perfusion (4.3 ml/kg versus 1.0 ml/kg, p = 0.005). Other parameters are evaluated, and considerations are made regarding adequacy of the dosage regimen. The drug seems to be safe and easy to handle in children.