Significant post-operative bleeding may occur following cardio-pulmonary bypass, sometimes necessitating early re-exploration. Two questions were asked by this study. Firstly, in children undergoing cardio-pulmonary bypass, can pre-operative, operative and immediately post-operative variables be used to predict post-operative blood loss? Secondly, in the same patient group, can the same variables be used to predict the surgical intervention of re-exploration for excessive bleeding within 12 h of surgery? In a prospective study of patients operated on in a 9-month period, bleeding fell rapidly in the first 3 h after surgery and early re-exploration for bleeding was necessary in 8 of the 206 patients. The only significant predictor of post-operative blood loss was minimum oesophageal temperature, low values being associated with greater blood loss (correlation r = -0.19 (P < 0.05)). With regard to re-exploration for bleeding, a long bypass time (P < 0.05) and abnormal pre-operative coagulation indices (P < 0.01) were significant and independent predictors. The data were also used to create a practical nomogram relating re-exploration for bleeding to hourly post-operative blood loss.