Hematologic studies were done on 145 children with congenital heart disease prior to open-heart surgery. Sixty-five of 111 patients with acyanotic-type disease showed some laboratory evidence of a preexisting hemorrhagic diathesis. Postoperative hemorrhage occurred in 40% of this group as opposed to 15% in the group with no demonstrable abnormality. Twenty-four of the 34 children with cyanotic congenital heart disease had abnormal blood studies and 10 of these had hemorrhage. Increased bleeding time with or without increased capillary fragility was most common in acyanotic types indicating either a microvascular abnormality or a qualitative platelet defect. Circulating fibrinolysin with decreased fibrinogen levels was seen in cyanotic tetralogy of Fallot. Twenty-two of 25 patients showed improvement in the hemorrhagic diathesis after definitive surgery.