The optimal red blood cell transfusion threshold for postoperative pediatric cardiac surgery patients is unknown. This study describes the stated red blood cell transfusion practice of physicians who treat postoperative pediatric cardiac surgery patients in intensive care units. A scenario-based survey was sent to physicians involved in postoperative intensive care of pediatric cardiac surgery patients in all Canadian centers that perform such surgery. Respondents reported their red blood cell transfusion practice in four postoperative scenarios: acyanotic or cyanotic cardiac lesion, in a neonate or an infant. In part A of each scenario, the patient was critically ill, but stabilized; in part B, the patient became unstable. Response rate was 58 % (71 of 123), with 45 respondents indicating direct involvement in postoperative intensive care. There was a wide variability in stated transfusion threshold, ranging from <7.0–14.0 g/dL for stabilized cases. There was no significant difference between neonates and infants in stated transfusion threshold. The mean hemoglobin level below which respondents would transfuse a stabilized patient was 9 g/dL for acyanotic and 11.2 g/dL for cyanotic patients, a statistically significant difference (2.2 ± 0.9 g/dL, p < 0.001). All clinical determinants of instability significantly increased transfusion threshold. Hemodynamic instability increased transfusion threshold by 2.3 ± 1.3 g/dL in acyanotic patients and by 1.3 ± 1.1 g/dL in cyanotic patients. Cyanotic lesion and clinical instability, but not patient age, increased stated red blood cell transfusion threshold. Significant variation in reported red blood cell transfusion practice exists among physicians treating pediatric patients in intensive care following cardiac surgery.